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  Multimeter specs
Posted by: Strelok - 05-14-2020, 03:16 PM - Forum: Test & Measurement - Replies (4)

Hello all,

I am in the market for a new multimeter, or how you call it nowadays.

Is True RMS useful for AC or is it just more expensive?
I'm thinking about a Voltcraft meter.

Kind regards,


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  Trying to trouble shoot Hum with LP PSU Pre
Posted by: Jonathan Levin - 05-04-2020, 03:24 PM - Forum: Personal Projects - Replies (7)

Hi all.

A friend of mine and I have been working on an amp that we built from the London Power F Pre Fender kit. The power supply is the PSU Pre power supply kit for tube preamp.

We finally got sound to come out of the set up and it does sound pretty good except we have some kind of hum. My friend is MUCH more knowledgeable about this stuff and this is what he wrote. I have attached photos as well. If it is any help, I can record a sound file to upload. but here goes. Thanks for your help:

"[font=Helvetica]Hoping you’re doing well in this time of CoronaPanic. We’ve built one of your kits, and we’re having a hum problem we can't diagnose or fix. [/font]

[font=Helvetica]We’ve combined the preamp and its power supply on a chassis along with a 50 wpc class D power amp and its power supply to make a hybrid.[/font]

[font=Helvetica]Here’s the specifics:[/font]

[font=Helvetica]– Hum is definitely on the preamp side. Power amp, when fed with different source, is dead quiet./[/font]

[font=Helvetica]– Everything is on a star ground except …[/font]

[font=Helvetica]– Hum is at constant medium level and doesn’t change when volume is changed. [/font]

[font=Helvetica]– When playing loud, you can barely hear it, but at low volumes it’s pretty prominent.[/font]

[font=Helvetica]– Hum is not pure 60 hz sine wave, but kind of dirty.[/font]

[font=Helvetica]– We experimented moving wires away from transformers, but no change[/font]

[font=Helvetica]– 12AX7; tried a new tube, but no change[/font]

[font=Helvetica]– Same with iron plate added above transformer as EMS shielding.[/font]

[font=Helvetica]– Only thing we added to preamp is a pot on the output as a level control so we can turn the input up and overdrive the pre at modest sound levels.[/font]

[font=Helvetica]– We bypassed the AC IEC filter, but no change[/font]

[font=Helvetica]– Measured voltages (let us know if we should measure anything else)[/font]
[font=Helvetica]- Heater: 17.4 vdc[/font]
[font=Helvetica]- Hi voltage, 367 vdc[/font]

[font=Helvetica]We’ve fiddled and twiddled, but we’re out of ideas. We’ve put a lot of work into this and would like to get it perfect. Hoping you have some suggestions for things to try. Pictures attached. Let us know if you want other shots."[/font]



[font=Helvetica]- - - Jonathan Levin[/font]

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  London Power's Power Supply for Preamps (PSU-PRE)
Posted by: London Power - 04-14-2020, 10:30 AM - Forum: London Power Kits & Mods - Replies (1)

Have you tried the PSU-PRE Power Supply Kit for Preamps? Post your comments or questions here!

[Image: psu-pre-new-1-297x300.jpg]

The PSU-PRE power supply kit for preamps is low-profile and will fit inside a 1U rack chassis. It uses a pair of DC, well-filtered, standard Hammond 229-series transformers to create the heater and plate supplies. The plate supply has two nodes that can be individually tapped with paired wiring. The heater supply can be wired for either 6V or 12V; 12Vr is preferred for use with London Power preamp kits and for using the ERK Electronic Relay Kit for channel switching. The heater supply also supports the switching circuits.

The regulated heater output can support up to two/four 300mA heaters at 6Vac or up to two/four 150mA heaters at 12Vac. There are three Links on-board that set the heater transformer wiring and regulator output to "6V" or "12V". The regulator keeps the heater voltage hum free.

There are two basic versions:
PSU-PRE-12 can support up to two 12A_7 tubes - enough for a 2-channel preamp
PSU-PRE-24 can support up to four 12A_7 tubes - enough for a 3-channel preamp

Read more about the PSU-PRE at .

If you're interested in London Power's tube preamp kits, see this page: .

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  London Power Electronic Relay Kit (ERK)
Posted by: London Power - 04-14-2020, 09:53 AM - Forum: London Power Kits & Mods - No Replies

Have you tried this relay kit? Post your questions and comments here!

[Image: erk-kit_sq.jpg]

The Electronic Relay Kit (ERK) features:

  • A, B exclusive selection
  • ground-referenced jfet shunt switch elements
  • silent operation
  • can be used with push-on push-off remote switch (not included)
  • easy wiring for panel control (switch not included – order our PSW)
  • can control panel LEDs (LEDs not included)
  • 6-poles included; infinitely expandable
  • use for channel muting or selection
  • use for gain selection via cathode-bypass caps
  • low current consumption of <2mA
  • operates from -12Vdc or high-voltage AC

This kit uses “shunt” switch elements for silent, reliable channel selection or gain stepping. Three transistors create and control two control lines that respond to a remote foot-switch. You can connect any required number of jfet switches to these lines. 

For example, you can use the ERK to select the channels of the LP-PRE Preamp Kit or the FPM Fender Preamp Mod by using one jfet on the control line to mute the ‘Clean’ channel, and two jfets on the second control line to mute the ‘Lead’ channel. Because jfet gates create almost no loading on the control lines, there is no true limit on how many jfets you tie on.

ERK has a fourth BJT to control one or two LEDs, or a bicolour 3-lead LED, to indicate which channel is ‘on’. If you use LEDs, ERK should be supported by a proper -12Vdc supply rather than the high-AC voltage.

ERK also has wiring accommodation for a front panel switch, which will be over-ridden if a foot-switch cable is inserted into ERK’s 1/4″ jack. 

More information here:

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  Tremolo Design Question
Posted by: makinrose - 04-07-2020, 05:04 PM - Forum: Preamp Mods & Design - Replies (4)

For some time I've been using this LDR circuit for signal varying tremolo: I've found it to be less noisy than the Fender version.  I've also used the Wide Range Trem that works well too.  I'm wanting to try a tremolo that wiggles the bias on a pre-amp tube by pulling in and out the cathode bypass cab like the Garnet example in TUT-2.  Could the LDR circuit I'm using be used to do that by hooking the negative end of the cap to it?  Before a start a prototype are there any design considerations I'll be needing to make?  Thanks Guys!

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  Mesa Boogie 50 Caliber+ Bias Cct. ??
Posted by: NGW - 03-17-2020, 12:12 AM - Forum: Power Supplies - Replies (7)

Hi All,

I have the misfortune to have a Mesa Boogie 50 Caliber+ (2 x 6L6) on my bench, with some rather dodgy Bias mods and various other examples of shoddy work.
I reluctantly took this on to help a good friend out and feel obliged to get it working properly for him.

The Bias cct. has been modified to incorporate an adjustment trimpot, which has a faulty track, causing the Bias to go from cold to way too hot with the smallest movement.
I can appreciate the motive behind the Bias mods, but the way they have been done is terrible.
I have removed all the components of this modified Bias supply and now want to make it work properly.
The problem is; I am not up to speed with Mesa amps, due to avoiding them, whenever possible.

Are any of you familiar with the Bias cct. and can you provide an accurate schematic? 
I have downloaded a couple of schematics from the web, but I'm not sure whether they are correct.
Can you explain this unusual (to me) Bias cct. and the Bias connection to the phase inverter cathodes?
What would be a good method of installing an adjustable bias cct., so the Bias can be set for non-Mesa branded (highly expensive) valves?

Thanks for any help.


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  Question about filament transformer for bias supply in a Power Scaled Amp
Posted by: makinrose - 02-06-2020, 02:05 PM - Forum: Power Scaling - Replies (2)

This might be dumb question....I'm typically using a DC elevated heaters like what is shown in TUT 3 and TUT5. If I need to run a filament transformer for the bias supply off of the heater supply do need to remove the DC elevation or use caps to block the DC?  Huh

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  YBA-1 bridged T circuit
Posted by: jmcd - 01-05-2020, 04:58 PM - Forum: Preamp Mods & Design - Replies (9)

I picked up a lovely Traynor YBA 1 that has a bridged T circuit between the second and third gain stages along with a plate-driven tone stack. I've read through the TUT3 chapter on the Bass Master, but I'd like to know more about the bridged T circuit (what it does, how it does it, and what sorts of component value changes are possible).

[Image: OjXgYt6l.jpg]

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  Tube Microphone Pre-amp Inputs
Posted by: makinrose - 12-25-2019, 01:32 PM - Forum: TUT Q&A - Replies (3)

I've been thinking about building some microphone pre-amps for recording purposes. Since the microphones I'm using are Lo-Z I was looking at matching transformers and at the cathode input circuit from TUT-2.  I've built pre-amps with matching transformers but have not had any experience with a cathode input circuit on a mic pre-amp before.  Are there advantages or disadvantages to that circuit versus a transformer?  Do they sound different?  Thanks guys!

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  50 watt amp project
Posted by: TiCatFan - 12-06-2019, 08:43 PM - Forum: Personal Projects - Replies (10)

Hey all;

looking for some help, i have put together the power supply kit PSU TPA and the power amp PA66, 50 watt version. i had the power supply working in a very prototype way, transformer and power supply board laying on my bench , but working. decided to add a power switch, it works, output transformer, power amp and front end. Hooked up an output jack and impedance selector, kept the options to a minimum at this point, master volume and presence control ( the ones available on the PA66 board ). Stuffed it in a hammond chassis, plugged it into the power limiting safety socket and turned it on, no tubes.

But, it doesn't work. I am trying to trace through the circuit to find the problem but thought I would ask on the forum if someone might know of likely suspects I should look at.

What I do know is, when I turn the amp on the light bulbs on the safety socket come on and stay on, fairly bright. The fuses do not blow, these are the fuses supplied with the kit and I am not sure offhand the wattage or blowage?? of the fuses. The panel LED does not turn on, nor does the 12v fan. Kind of afraid to leave it on long enough to check for voltages anywhere, I have left it on for a 10 count a couple of times, nothing hums or smokes or anything.

Is it possibly a messed up MOSFET?, any ideas on what to look at and how would be greatly appreciated. I have been trying to follow the AC supply but I also wonder if a short to ground would take out the fuses. What might draw power but not blow up the amp???

Greatly appreciate any help I can get...


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A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
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