Hi everyone.  I'm looking for some input.  I had a custom-built amp come in the shop that essentially is one channel a 60's Fender Bandmaster with a three-tube spring reverb unit in front.  The transformers are a Super Reverb clone for the power and Bassman clone for the output.   The thing I'm concerned about is current draw on the high voltage secondary since the amp is now powering a 6K6 power tube in the reverb portion as well as the 6L6GCs.   

The power transformer has rating of 300 ma for the 325-0-325 secondary.  In a Super Reverb with a 5U4GB about 248 ma would be drawn leaving a healthy safely margin.  My best calculation is that this circuit would draw about 270 ma.  Is that enough of a safety margin? The 6k6 is drawing about 30 ma.  Should I be concerned?

Hello, TuTians,

I wasn't exactly sure which category I should post this question, but anyway, this is a question about a phenomenon that I have rarely seen addressed in guitar amplifier forums. And that is:

Which components or stages of a guitar amplifier should I direct my attention to, for modification or parameter tweaking, that would have the most success in enhancing the phenomenon of having that entirely desirable sort of spontaneous, sustaining feedback, that can happen while you are playing, that seems to be a natural by product of SOME certain rare amplifiers? What is it that I can change or modify about a circuit in a tube amplifier to build this kind of interactivity with the guitar into an amplifier? Is such a thing even known?

I KNOW that this kind of playability in an amplifier is possible, within reach, and NOT built with some special kind of "un-obtanium," because I had the opportunity to play through such an amplifier, ONE time. This was when I was in a studio during some recording sessions, in which I was tasked with dubbing in a little solo part into a section of a song. I only had my guitar with me that day so, the engineer just grabbed one of his friend's heads that happened to be at the studio that day. When I plugged into that amp, the tone completely floored me. I wasn't even ready for it! 

No. seriously. When I plugged my guitar into that amplifier, hit a chord to check for tune and then played a couple of notes, just merely bending into vibrato, the thing CAME ALIVE!  How easily it would just sing... and sustain perfectly into these notes that were some portion of the harmonic series, that seemed to work with every note I played. I was so astonished at the incredibility of the tone from that amplifier, that I actually DID NOT get to play on the solo in that song that day. 

I know this sounds silly, but every time I tried to do a take, I got completely distracted by this amazing electric tone that I had never heard coming from my guitar before. So much so, that those emotions totally devolved my playing into nothing but fits of laughter and Tourette's Syndrome like cursing, just stuck in postures of complete exasperation, high-fiving nothing but air; like some over-excited Patriots fan watching some epic touchdown. Embarrassing. I simply couldn't keep it together. And because time is money in a studio, after about 3 or 4 tries, I got quickly switched out to give someone else a try on the solo. (Which he nailed, by the way.) 

I'm sorry for such a long dramatic anecdote of my first time with a really great amp, but up until that day, I had NO IDEA that kind of tone was possible in a guitar amplifier. Of course, you guys will relate, because that kind of feeling is, after all, why we are all here. So, I guess what I am asking is, what do I need to do to get, build, modify or acquire an amplifier like that one? And I'm talking about getting that singing spontaneous feedback, that would just miraculously appear at the end of a run.

The gear I used couldn't have been more straight forward. That is why I KNOW that tone was ALL the amp. I had my own guitar with me that day, which was an all stock 1996 Japanese Fujigen made Fender, '68 reissue Strat in naturally finished swamp ash, and a maple neck. This guy's mythical amp that I plugged into was a non-master volume, non-plexi (i think), '68-72 or so, 100w Marshall Super Lead head, plugged straight into an older, slanted Marshall 4x12 cabinet. The engineer told me that the amp WAS modified in some way, but he didn't know how. And curiously, I WAS NOT playing at any deafening kind of level or even really loud at all.  

He had grabbed that amp for me to use because it was very similar to my OWN, which was a stock '69 100w Marshall Super Bass that I plugged straight into a vintage 30 loaded slanted Marshall 4x12 cab. But it didn't sound like that!

So, can that kind of feedback come from any type of amp? If so, what kinds of things should I focus on in an amp build, or WHICH KIND of amp build should I target in building to get that kind of feedback? Does it have to be a Marshall style circuit? I do love that Marshall kind of tone, by the way. Could I get something like that from a modified or tweaked 18w clone or a JCM800 micro kind of kit, or something else? 
(Am I playing 30 years and still an amp noob?   )

Thanks for ALL of your kind help, 

Scott

Hi I'm interested in knowing a bit more about the Z Pre-amp design.  From the description it says it's Soldano inspired?  I'm assuming that it does away with the cathode follower to lower the tube count and make the EQ more effective?   

I'm looking to be build something with the gain level of SLO-100 but smoother and fuller and sweeter sounding---a little less fizzy than the SLOs I've heard.   Will the Z-Pre amp work for that sort of thing?  Are there any suggestions about places in the circuit to tweak?  I'm interested to hear people's experience with it.  Thanks everyone!

Hiya!

I have access to a CNC router table at the local makerspace, and realized it can probably cut down on the time for me to fabricate some cabinets. The one thing I'm unsure about is the finger-joints. Assuming I'm only able to work with the bit perpendicular to the sheet stock, I'll have to cut "dogbones" for the internal corners of the finger joints (picture from here https://cutlasercut.com/getting-started/cnc-machining/):

 Now I've seen that there are ways to minimize their visibility (https://fablab.ruc.dk/more-elegant-cnc-dogbones/), but I was wondering if anyone has any strong opinions about dogbones in general for finger joints. Effects on strength, durability, air-tightness of cabinet, extra work to fix these issues, etc.

Thanks!

Hi Guys

Most modern solid-state power amps use symmetric supply voltages, commonly referred to as "split rails". This just means there is, say, +/-50Vdc supporting the circuit. In absolute terms each rail is the same voltage, only the polarity is different.

The supply voltage has to be sufficient to accommodate the peak signal amplitude into the speaker load at full load. So, the +/-50V will accommodate a 40Vpk signal into 8-ohms, corresponding to 200Wpk or 100Wrms. There is 10V extra, which accommodates losses over the transistors and current-sense resistors (emitter resistors). If the supply stays solid at 50V at full load, there should a few more volts possible into the load as we do not want to rate the power of the amp right at its limit lest it may not reach that value under low-mains or other conditions. On any case, the supply rails need to be in proportion to the power output desired.

Most PA circuits live within the same supply voltage as the output stage requires. However, there are some designs where the front-end circuitry requires a higher voltage, and these higher rails can be created in various ways. The two broad methods are supply stacking and having independent sources.

Supply stacking is exactly as it sounds: a second supply is stacked on top of the main supply. Where a typical PSU has a center-tapped winding, a full bridge over the whole winding and filter caps and bleeder resistors from each of the bridge outputs to the CT, the stacked supply adds another small supply on top of each rail. This smaller supply is often a small OPT with dual secondaries, where each secondary has a full bridge across it, followed by a cap and bleeder resistor. These small supplies are identical and one has its negative end tied to the positive main rail, while the second small supply has its positive end tied to the negative main rail. We end up with V+HI, V+, CT (0), V-, V-HI. Note that the V+/-HI are relatively low-current and would not be suitable in a class-G or class-H amplifier.

To size the small PT, we look at the current draw of the front-end circuitry. The typical differential input stage draws constant current. The transimpedance stage may have a constant current source for one side, but the other side may draw more current under load. Predrivers or driver stages draw modest currents but can sink and source much higher currents, so these should be assessed in simulation or in a real circuit. How much current the small supply must support depends on the circuit load, and we most often have drivers and predrivers supported by the main supply, alleviating some of the variable loading on the small supply.

We also consider the nominal voltage that the front-end circuit operates from. Because it sits atop the main supply, which will sag under load, we have to account for that sag plus any sag that may occur due to low-mains to both PTs. This generally means that the small supply voltage might be twice the difference we need between the idling main rail and the idling front-end rail. For example, a mosfet amp might have +/-65V main rails with +/-75V for the front-end. We intuitively view this as needing 10V boost supplies. If that is all we add, then as the 65V sags, so too will the 75V, and this may not be good for the stability of the quiescent point for the output stage, or possibly for the overall circuit stability.

If we double the boosted supply to 20V and add a ground-referenced regulator to provide 75V, then we accommodate up to 10V of sag for the front-end. The reality is that we need a bit more sacrificial voltage here.

As a reference,the inherent regulation of typical toroidal PTs is quite good, varying with the VA rating:
80VA   10%
120VA  9%
160VA  7.7%
225VA  7.2%
300VA  7.1%
500VA  5.1%
625VA  4.1%

Some front-end circuits incorporate active current sources while others use resistive ones. For the latter, a regulated supply maintains bias stability, but means the amp might be unstable while powering up. As mentioned, the regulator must be ground-referenced and it is simplest to use a discreet circuit for this. For active CS circuits, we may still wish to regulate the voltage, or at least have an active hum filter for each boosted rail.

The PT for the boosted supplies may be quite small depending on the circuit, maybe 10-15VA, but could be up to 50VA for a large amp or when significant drive currents must be supported by the boosted rail. Each boost supply is derived from one secondary, so whatever the current and voltage product is for one boost supply, we must double it for the PT VA rating.

The small PT VA goes up proportionately if we use it to create fully independent high rails that are ground-referenced. The circuit is identical to the main supply, just a smaller PT with higher voltages.

The boost supply PT can be a small toroid, semi-toroid or EI type. EMI from this PT should be low as the load is balanced over the secondaries and fairly constant.

Have fun

Hi Guys

Some of the kits will be mounted in a specific way, where others have a very usual way, and others can be mounted various ways.

The first group includes:
Power Scaling kits
QS kits
VCK

The second group includes:
PSU kits but not RBX
MICRO
PA50H1

The third group includes:
preamp kits
PA66
PX66
PAF
TREM which can be zip-tied to a wire harness

Card guides are typically used in computers where there are standardised board slots where the accessory cards are of a prescribed shape with edge connection "fingers" that fit into a connectors on the backplane card. usually the metal front of the card is mechanically secured to the computer case once in position.

In audio, such a scheme is never used.

The trickiest is mounting the preamp PCBs, PAF and the PA66 variants, which can be mounted parallel to the chassis or perpendicular to it. Parallel mounting uses spacers, bolts and lock nuts, or a spacer and two bolts per hole position. For the internal and external mounting of the PCB as shown in the info sheet supplied with the kits, these hardware approaches are used.

If you are mounting the preamp board in a 1U chassis, then an internal mounting requires the use of angle brackets. If you mount the board on the rear panel, then it can follow the internal/external info sheet.

As far as typical hardware goes, you can get standard 4-40 threaded spacers with 1/4" hex or round shape - hex is easier for tightening, and use 4-40 machine screws. The spacers are standard Keystone parts that you can obtain from Digikey or Mouser. If you want the tube to stick out maximally, then a 0.375" (2202) to 0.5" (2203) spacer will suffice; if you want a modest stick-out, then a 1" (2205) spacer is good leaving enough of the tube out to be easily removed while providing good protection.

Have fun

Hey fellow TUTians,

This is my first post ever here, and thank you for taking the time to peek at my question.

MY QUALIFICATIONS/SKILLS (you can skip this part): I am basically a very enthusiastic and adventurous amateur electronics repair and build enthusiast. I have tinkered with fixing and morphing electronic gear for myself and bandmates throughout my years. I have a small working set of test gear and a solder station. I have mostly been educating myself through reading various books, manuals, websites and forums. I'm getting much better at reading through schematics. I feel like I am ready to up raise the difficulty of my project levels.

I have a little 20w McMartin PA head that is wired point to point that has 2x 6L6, 2x 12ax7, and 1x 6av6 tubes. This is my first attempt at trying to convert a PA into a Guitar amplifier. This attempt is an intention to both sharpen my electronic skills and hopefully end up with a killer little tube amp in the process. The project is a little bit above my pay grade but not drastically so, and besides, there is no greater teacher for an ambitious mind than 450v and an idiot that just feels lucky. (That's a joke)

What I could really use some help with here though is possibly some coaching through this process. (you don't know how difficult it is to find mentors once you reach the age that you start to look like a mentor.)

Some questions I have right at the outset mostly concern with trying to figure out the best direction to steer this carcass into, and which direction makes the most sense as per guitar amplifier designs that I could use for inspiration to try to model:

Here's the schematic:


  McMartin MA-20 Schematic GOOD.pdf (Size: 43.17 KB / Downloads: 5)
and some pictures:

           

1) what does an expert notice right off the bat that this amplifier's design is telling you when you see the physical layout and read through the schematic?

2) We have to start somewhere, right? And there is no better first step than arriving at establishing a goal or endgame for our project. So, looking at the schematic and considering the transformer voltages and tube compliments, etc. what type of known good guitar amplifier design could be most easily cloned or used as a model for this build? 

3) There are three (extra) tube sockets on the chassis for mic and line transformers. Does having those sockets already built in and ready for use change or expand the range of tube amp models we might like to try to emulate with this build?

4) I understand that this amp, being intended for PA use, will have been designed with the intention of avoiding the kind of distortion characteristics that are most pleasing for guitar amplifiers, so what types of changes would need to be addressed in order to make the conversion?

5) What logical steps should we take in assessing the schematic, and then planning our approach to hashing out the methods for accomplishing this mod?

6) Is it a best practice here to piece through this mod by breaking up the schematic, referentially, into separate systems that we should work on solving, one at a time, in order to keep our goals and tasks focused and with clear endpoints for each of those?

7) Should I break this down into DC Signal path, Input section, Gain stage 1, stage 2, power tube amplification, output section. AC Power section, etc.?
 
Well, I was going to ask a ton more questions, but I think I should probably take my time and try to get the full benefit of having expert advice at my disposal, and would be best served by letting more knowledgeable eyes guide me that can lead me through a project like this so that I can somehow through osmosis or some other such wizardry, absorb some of your wisdom and logical methodologies in trying to adopt them for my own.

Well, OK, but there are just a couple of questions right off the bat that I know would be further down the road but that I find irresistible in inquiring about right now:

1) the sections of the amp that were intended for plugging in transformers for the microphones in the input sections: what will most likely become of those extra tube sockets? If I were planning on just eliminating those sockets from the whole build, would I just remove most of those connections to those sockets and then run through the schematic one section at a time trying to figure out where each connection should go to in following my new plan?

2) could those now empty and not needed mic trans tube sockets be used as potential opportunities to build in other new gain stages, like for reverb or tremolo?

Thank you to any that could offer any help to me for this kind of thing, I really appreciate it.

Scott

Hi Guys

My own toroidal transformer guru retired a while ago and that has caused me to think about what direction I want to take with my amp line. His rates for custom power and audio transformers were very much on the low side and he did not penalise you for prototypes or low quantities.

Building preamps is not a problem as there are many suppliers with appropriate off-the-shelf products that are suitable. It is really the support of power amps that is tricky. You can find good output transformers but the PT designs I have seen all leave something to be desired.

Note that when using a toroidal OT it MUST be FULL-POWER-BANDWIDTH. You CANNOT use the OT to restrict the frequency range as it will overload in an extremely non-musical manner.

The nearly universal problem with toroidal PT offerings is that there is no proper bias winding.

In a tube power amplifier the bias supply is the most important voltage in the entire circuit.

As readers of TUT (The Ultimate Tone) know, a separate bias WINDING on the PT is preferred in the method for achieving lowest-noise and to properly support a Power Scale solution. A bias TAP on a center-tapped plate winding is workable, but far from ideal.

Toroidal transformers are nearly ideal in their functioning and because of this we have to specify them and use them a bit differently than all other transformer types. This mainly manifests itself in how we configure rectification inasmuch as half-wave rectification should be avoided at all cost. This means that a bias TAP is not a good idea AND that the basic CTed plate winding is also not appropriate unless you are in fact using a tube rectifier - there are other ways to incorporate a tube rectifier which use a non-CTed winding. As TUTs point out, the CTed plate winding is really just two half-wave circuits over-lapped and each winding has to be able to support the full load - a real waste of wire and winding space. The discontinuous conduction of half-wave rectifiers injects noise into the PT, which couples into all the other windings and back to the mains. The efficiency of toroids makes this a much worse situation than with traditional transformer designs.

So, toroids need to be specified with windings that require a full-wave full-bridge rectifier, usually referred to simply as a "bridge rectifier" and often attained as an integrated 4-pin package.  The winding is used to support the load continuously over the full AC cycle of the mains to minimise EMI. The plate winding should be like every other secondary inasmuch as it is a single winding with no CT.

The plate winding may have a tap to allow different maximum output voltage using either a switch or hard-wiring. As TUT4 illustrated, this tap will not provide an alternate but lower voltage at the same time as the full winding is being used, nor the reverse, where the full winding cannot provide a higher voltage while the tap is in use.

The capacitive-coupled bias supply some manufacturers use with the proper plate winding is clever in all the wrong ways.  It has a high-ish impedance and imposes a discontinuous load on the winding, injecting nose. It has all of the wrong characteristics to supply what is the most important voltage in the entire chassis and will not support a bias regulator. It is truly a false economy.

In my survey of toroidal PT offerings for tube amps, I see the CTed plate winding BUT no 5V heater winding for the presumed tube rectifier?  The use of such windings continued well past the reign of tube rectifiers, and this can only be due to lethargy on the part of both the amp builders and the transformer suppliers - an unspoken agreement. In any case, many of the toroidal manufacturers today are newcomers to the industry and it must be assumed they are simply unwittingly copying the mistakes of legacy manufacturers without realising it. Or, they have some gaps in their understanding about tube power amps?

Heater windings tend to be plentiful to over-abundant in modern toroidal PTs for tube amps. Most have at least two 6v3 windings with many providing a CT to at least one of those. The CT is fairly useless since a faux-CT works as well, and in audio the use of a DC-stand-off for the heater winding is inexpensive and provides as much hum rejection as  DC heaters but without all the trouble and wasted heat.

You have to remember that with a toroid EVERY winding must fully cover the core to help shield it, which means that it must be made with enough wire to wrap around the full circle of the toroid and end right beside where it began. A CTed winding is in fact two separate windings that each must wrap all the way around the core, and then have the appropriate leads tied together and brought out as a center-tap. Doing this for low-voltage windings imposes some difficulty upon the manufacturer. The wire lead-outs impose unavoidable gaps in the shielding, so minimising the number of these is helpful in reducing EMI.

TUT4 presented London Power's 6/12 heater system. This uses a 12V CTed winding to supply a mix of 6V and 12V heaters. For lowest-noise from wiring, using 12V for preamp tubes that accommodate it allows the triode leads to never have to cross the heater wiring. We can do this in both hand-wired and PCB layouts. In a power amp, the 6V (octal) tube heaters can be distributed on either side of the CT (and with separate wire runs can use #22 wire). These loads do not have to be equal about the CT for the reasons cited above. With many of the tube amp toroidal PTs on offer, the two 6V windings can be wired this way and tied to a DC-stand-off. If one of the 6V windings has a CT we simply do not use the CT.

So, most of the available toroidal PTs for tube amps have no bias winding and have no 5V winding to support a tube rectifier. The latter is not really a problem for most builders. To have a proper bias supply, we end up adding an auxiliary supply of one form or another to create a bias supply of sufficient voltage and impedance.

In the US, Antek has a range of PTs and OTs
https://www.antekinc.com/transformers/

In Poland, Toroidy has PTs and OTs, serving all of Europe
https://sklep.toroidy.pl/en_US/index

There are certainly many other companies providing off-the-shelf devices for hobbyists and builders.

Note that the information for each device is often scanty and there is no "match-up" suggestions of PT + OT.

There are often a range of finishes from standard tape-wrapped, to available cans, and then potted centers or potted in a can.

Hi Guys

Vacuum tubes aka valve, allowed the electronics age to begin and grow into something indispensable in our modern lives. For most things, newer technologies have taken over from tubes, but there are still places where tubes are either the only possible gain element to use, or they are simply preferred for their unique characteristics. For example, microwave ovens use a magnetron tube to generate the microwave energy used to stimulate food to cook it. Add a wave guide and a capacitor and this is the heart of every microwave oven regardless of what the front panel looks like. Very high-power radio transmitters are still tube-based although modular solid-state system are working their way into this realm.

For audio amplification, tubes are still used by many manufacturers, hobbyists and music enthusiasts even though dramatically lower distortion can be attained using solid-state circuits. Nominally, a playback system is not supposed to change the sound, but every listener assembles a system to his own liking, and this is because our individual hearing is unique as is our aesthetic sense of how things should sound. Tubes are considered to be almost "organic" in how they handle music, which can be explained in scientific terms of their structure and resulting nonlinearities. The varying transfer curve of what the output looks like for any given input level gives the tube a "pleasant" character, and makes it doubly useful for musical instrument applications where the amplifier is "the other half of the instrument".  MI is about tone creation, so we want the character of the tubes to shine through, and to be able to combine these characters to achieve our sonic goal.

Fortunately for both hifi and MI, there are a lot of power tube types that share a common base type and pin-out, making them more or less plug-and-play compatible - with the necessary check of the idle condition to assure safe operation. The sound of each tube is unique, but we can group them all based on their sonic texture, which strongly follows their internal structure.

The first broad strike of distinction of power tube tone is based on the use of beam-forming plates. This was an RCA development in the 1950s, and it resulted in a reduction of distortion contributed by the tube. Tubes using beam-forming plates are still called "pentodes" or "tetrodes" as there are still five or four elements within the tube, respectively, but the screen grid is replaced by plates. There is no need for a critical alignment of the various grids, and the electron stream is focused towards the plate.

Beam-forming plate tubes:
6CA7
6L6GB/GC
5881
6550
7027
7481
7491
8417
These tubes have a neutral tone with no specific emphasis of frequencies. Compared to the types below, this group may sound "clean", or "less bright", or "less harsh".

"Kinkless tetrodes" is Mullard's term for their power tetrodes with critically-aligned  grid wires. This group includes the famous KT-series:
KT-66
KT-77
KT-88
KT-100*
KT-120*
*Devised well after Mullard went out of business and the trade name was bought by Mike Matthews of Electroharmonix. So, the structure may not be true to the legacy?

Kinkless tetrodes tend to have a higher distortion than the beam-forming plate tubes, with this distortion being perceived as "thick", "rich", "brittle", or "harsh", "muddy". Personally I find this group to have a muddy tone, but it is a huge component of the distinctive Hiwatt sound.

Structural pentodes have grid wires for the control grid, screen and suppressor. As a power tube there is some critical alignment necessary. This group includes:
6BQ5
6V6
6973
EL-34
EL84
These tubes tend to have a "brittle", "bright" or "harsh" tone, except for 6V6 which is either  "creamy" or "muddy". Being a 9-pin miniature base type, the 6BQ5, EL-84 and 6973 have no substitutes per se, as all have the identical tone.

Each group has variants of many of the tube types within it and there are no doubt other tubes not mentioned here.

Note also that the tones described are for the wiring connection using the tube elements as generally perceived to be "standard", where signal is applied to the control grid, the screen is tied to a fixed voltage, and signal is taken from the plate through an output transformer. The character of each tube will change when alternate operating modes are used, such as triode wiring or the use of ultralinear taps on the OT.

Triode tone is generally perceived to be "clean", but can be deemed "dark" when compared to a reference tone that contains more distortion (as with KTs or structural pentodes / tetrodes). Structural triodes and heater/cathode triodes have a tone similar to the beam-forming plate pentodes.

Ultralinear tone is somewhere between triode tone and the innate tones of the various tube types. UL connections are intended to reduce THD within the tube and the OT yet exhibits a compression effect of its own, representing a gain nonlinearity with signal swing. For most musicians, Fender's dabbling with UL connections turned many players and hobbyists away from this operating mode, but their sonic achievement resulted more from other factors of the rest of the circuit design.

In hifi, many of the "tubes with character" are revered, and this depends a lot on the circuit the tubes are used in and the interactions between the power amp in total with the speakers. Many hifi PAs use unstable circuits with too many gain stages within a single feedback loop. The classic Williamson falls into this category despite achieving good results for the day. A design that squashes the tube characteristics to achieve remarkable results is the MacIntosh Unity-Coupled OT design. Both Williamson and MacIntosh paid great attention to the winding of the output transformer, with the latter being a break from tradition combining both plate and cathode drive, with the bonus of bootstrapping the screens and the driver tube. Audio Research followed along in the plate + cathode drive and often used cathode feedback to the output tubes to provide better linearisation.

MI power amps using tubes fall into two basic designs based on the splitter circuit used. For both, the overall effect of a net low-gain means that the character of the power tubes is well out in the open, and substituting other tube types lends different textures to the amplifier. This means that the player does not have to be limited to the tone of the stock tube set, provided there is a means to adjust individual tube bias (as with our Bias Mod Kits).

As TUT3 demonstrates, the famous Ampeg SVT (Super Valve technology) and V9 amps used an unstable, overly-complex power amp circuit that required many band-aids to make stable. Fender copied this circuit for use in all of its current 300W tube bass amps. As is the usual result of such copying, all of these amps tend to "eat" power tubes unless we make the simple modification recommended throughout the TUT-series.

Hi!



I was wondering if anyone had any thoughts on the "new" master volume that Friedman and Suhr amps seem to be using, as seen on the Plex and SL68 amps. Bias grid-leaks are replaced with 2M2 resistors, and each resistor gets one gang of a dual gang pot (each gang wired as a rheostat) in parallel with it. As you turn down the pot, you reduce the grid-leak value and load down the phase inverter, or such is my current understanding.

I'll post a sketch after some sleep.

Thanks,
          physics

Edit: sketch