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  Bass Master and compressor
Posted by: sinusoids - 07-17-2020, 06:36 PM - Forum: TUT Q&A - Replies (6)

I'd like to build a TUT3 Bass Master with compression.  Would the TUT2 fig 6-54 Simple Transformerless Compressor work for this application?  Following the tonestack ?

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  Tremolo... a Project To Be! Or not..
Posted by: Strelok - 07-08-2020, 08:59 AM - Forum: Personal Projects - Replies (8)

Hi all!!

Did any of you build the Fender Tremolo in TUT2?

I like the idea of frequency changing with the sweep a lot.
But the LFO made of tubes may not be very efficient, an OpAmp would be much cheaper and smaller.

I have no idea what voltage or current the tube produce to create the sweep.
Maybe I will build it on my prototype board.

Is it worth building thios thing or is the sound just "average"?


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  Distorting reverb tube
Posted by: Tomislaw - 06-26-2020, 03:28 PM - Forum: Preamp Mods & Design - Replies (16)


I'm having a bit of a problem troubleshooting a Fender type reverb - the 12AT7 driver tube output has a tendency to distort the signal. Originally with a bypassed 1K cathode resistor the tube was oscillating wildly. I increased the Rk value to 2,2K bypassed which stopped the oscillation but the top of the signal was still distorted. Removing the bypass cap obviously reduces the gain in the triode and I can make up for that by lowering the Rk value to 470R. At maximum V1 output volume I'm getting 154Vpp signal swing whereas with a bypassed 2,2K Rk it was around 195Vpp. At 154Vpp reverb sounds OK. Voltage drop at Rk is now 4,7V and plate voltage measured cathode to plate is 340V, so the plate dissipation is still within spec.
The signal at the grid is clean and undistorted. There's a 430K grid stopper and a 680K grid leak. Reverb transformer sees 360V.

Is there any other way of fixing the problem with the reverb driver?


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  Dopplesonde fixed bias
Posted by: sinusoids - 06-06-2020, 10:55 PM - Forum: TUT Q&A - Replies (13)

A. I built a pair of Dopplesondes with 5F6 preamps.  All is operational except the fixed bias.  As built, it had -20V B-.  With Cathode bias switched in, B- was -340V.  After trying lower R for the Bias supply I found with only 56k R, the bias was -94V with one set fixed and-56V with both sets fixed.  I now have one 56k R and 22uF C dedicated per tube set.  Both tube sets now have -56V max bias each.  They still interact quite a bit.  I can set bias for both V3+4 and V5+6 but if I have both in fixed bias the idle current for both increases quite a bit.  Any guidance here?  These current values include a switch to the grey 115VAC PT input tap – originally had used the black 125V tap.

B. Is my TUT bio editable?  I did not know that info would be included with every post – can I delete the bio for new posts?

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  Applying a SUS control in a SE Amp with a Master Volume
Posted by: makinrose - 05-30-2020, 02:05 PM - Forum: Sag & Sustain - Replies (4)

I'm trying to figure out the best way to apply a SUS control to a cathode biased SE amp I'm building.  Overall the topology will be similar to Blackface Champ but it will have Power Scaling and I'd like to add a SUS control.  For the limit control I was planning on adding a Master Volume before the grid stopper of the power tube.  Can the SUS circuit connect where the Master Volume pot usually grounds or is it better to have it work off a grid leak resistor after the master volume control?  Thanks for the help!

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  Stand By and Slow Heater Turnon
Posted by: Strelok - 05-27-2020, 08:36 AM - Forum: Power Supplies - Replies (5)

Hello all you happy people,

What's the deal with Stand By nowadays?

I read many things in Kevin's excellent books.
If "cathode stripping" is an issue than I should have those switches in all my preamps too!
Or does that only come into play with amps of "high voltage", which I learned is originally 1000V+ ?

I'm building a Jim Kelly amp like in the project, and for geographical reasons (the layout of my PS that is) I woul dlike to have that switch after the first resistor.
That should be no problem I think?

Is the strain on the switch not too much?

Also, I saw an excellent schematic for building a slow power-up for the heaters.
Is anyone doing this?
Is this schematic only meant for DC operation?
I'm asking this because I remember a BJT there.

Kind regards,


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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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Come in where it's warm!
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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