6SN7 Push Pull Reverb Driver

[didit]

Hello -- This falls well within the clear established definition of fair use** ..

**https://en.wikipedia.org/wiki/Fair_use

[martin manning]

Might be ok to post a snip like that under fair use?

Anyway, here is a simulation of the circuit. Looks like it will drive 33mA RMS into the 8 ohm load with 2V peak input, 400V B+. Everything looks to be within limits.

edit: completely disconnected FB NW and adjusted OT DCR, fixed Lpri

[W5FH]

6SN7 Push Pull Reverb Driver.jpg

Thank you all! You guys are the greatest! I really appreciate the tech help here as I am trying to understand more about these circuits. Here is a handwritten schematic of my 6SN7 push-pull reverb driver using a 2-watt MPS 22.5K to 8 ohm transformer. Would it be safe to raise the B+ supply to transformer center tap from 322 to 400 vdc? I assume my ouput will increase some.
The reverb is close to where it needs to be according to player but the Dwell pot ahead of .002 input cap is being ran fairly high.
I know I have created some confusion here between the two different topics. Last year built a close copy of the Wonderland based on the schematic (Two Rock John Mayer Signature) I posted which is called the Fairfield County Signature. This made a nice sounding amp. Recently I built an octal version of the same amp but I used a Fender TMB tone stack leaving out boost switches and I left out the little local feedback networks. I tried to layout the 5 octal preamp tubes in a logical manner to avoid a lot of interconnecting signal leads. The mixer tube is in center of the row of 5 octals. I used a 12AX7 LTP phase inverter as it was a little tight for octal in that spot. This way the mixer goes right into P/I.
What surprised me was seeing the same effect (grid pick-up of hum) in the second dry stage triode in both of these builds. That second dry stage is V-1B in the 12AX7 version. The grid is fed by a 470 pF cap in parallel with 220K resistor. Both in the 12AX7 and in 6SL7 version the amp would not get dead quiet with nothing plugged in. There is a very tiny hum. If I ground the grid directly with screwdriver the amp is dead quiet. If I ground the other end of the resistor/cap network the hum reduces but only slightly. In the 12AX7 version I had a center tapped filament winding and applied about 70 vdc elevation voltage to it. In the octal build that UTC hermetically sealed power transformer did not have a center tap on 6.3 volt winding. I used a 500-ohm pot with 91 vdc elevation voltage on pot wiper. This humdinger has a very good null but cannot get amp dead quiet. I was just baffled by how this same effect showed up in the octal build.
Again, sorry to confuse things here and I am very thankful for all of your help!!
Byron W5FH

[sluckey]

This is Merlin's circuit redrawn using Visio...

[martin manning]

Here is a handwritten schematic of my 6SN7 push-pull reverb driver...

If the B+ in the simulation is reduced to 332V the rest of the voltages are within ~1V of your measurements. As noted above going to 400V B+ shouldn't be a problem.

[Helmholtz]

The Hammond 125A has a full primary inductance of 14.94H.
So the inductance per primary half is 3.735H (half turns - fourth inductance).
That changes the voltage step-down ratio by 1.414.

[martin manning]

Fixed. See new circuit and plot.

[W5FH]

Again, MANY thanks to you great group of fellow amp builders! I have attached the data sheet for the little push pull reverb transformer that I am using. Just running it by you guys to make sure it is adequate. I was a little crowded for room and wanted to use a little bit larger unit. I am powering the center tap through a 7500-ohm power resistor off of a 415-volt bus. I will lower the value of this resistor to have 400 vdc at center tap. I believe a 1.2K will get me close.
This is my first build using a push pull reverb driver. The reverb in this octal Wonderland has a very nice tone. I am pleased with the amp with exception that it is not quite dead quiet. I am using ECG/Sylv 1986 6188's in place of 6SL7's. I like these tubes. My power supply is choke input which is not the norm for guitar. I had this UTC pair power transf/choke hermetically sealed out of commercial surplus. The B+ at output transformer center tap is 425 vdc with 6L6GC's idling at 45 mA cathode current each. The output transformer is the K-70272 (4K pri to 4,8,16) out of a Conn organ that used a pair of 7027A's. This is a nice O.T. with good specs. My bias set-up for output tubes has a bias level and bias balance adjustment. I can get 44 watts out before clipping with a 9 mV input RMS sine wave.

OT2PP_USA_DWG_STD (3).pdf

[Helmholtz]

Fixed. See new circuit and plot.

What output voltage across R7 are you getiing now with an input of 1Vrms?

[W5FH]

I meant to ask Mr. Steve (sluckey) -
I have two unused 6SL7 triodes in this build. The input tube V-1 has only one triode in use, driving the tone stack. I did this deliberately in order to convert to a parallel input stage if desired. The other unused triode is in V-4, this tube is amplifying the signal from reverb pan. Do you think I could get away with using the unused triode in this tube V-4 for your Tremonator? I had made a note about this on my hand drawn layout sketch. I have incorporated your Tremonator in many of my builds. Hats off to you!
Byron W5FH

[martin manning]

What output voltage across R7 are you getiing now with an input of 1Vrms?

1V rms Vin at 1kHz produces 188 mV Vout

[Helmholtz]

1V rms Vin at 1kHz produces 188 mV Vout

Thanks, so a total gain of 0.19.
About 20% lower than my original estimation of 0.23.

[martin manning]

Playing around with the simulation I don't see any great change going to 400V B+. The signal level only increases by a few percent. For reference, load lines for 300V and 400V (first triode) are shown below on 6SN7 plate curves. These load lines are drawn from the simulation results, where the line shown passes through the long axis of the elliptical path. For a little more sensitivity you might try biasing a bit hotter at 300V B+, using a 390R 0.5W resistor in place of the 910R, biasing at about -6V as shown below. That puts the operating point up where the plate curves are a bit more spread out. Output voltage for 1v RMS input increases to 213.6 mV.

[W5FH]

Martin-
I cannot thank you enough for taking the time to help me here! This is exactly the info I was looking for. I have both Merlin Blencowe books and in the latest book he details how to draw load lines for push pull stages. I was struggling with trying to do it and sure needed this help. You are a real blessing to us amp builders!
Byron W5FH

[W5FH]

I changed the cathode resistor from 910 ohms to 430 ohms and left the dropping resistor alone for this 6SN7 reverb driver, it is a 7.5K coming off of 415 v bus. At idle I measured 301 v on center tap of output transf. Showing 6.7 vdc difference between cathode and grid resistors. Looks to be drawing 12.7 mA at idle. Test drove the amp and can easily tell the reverb tank is being driven harder. Can get close to cavernous at full dwell. I believe I am where I need to be with this reverb driver. Again, I am in deep gratitude to all of you great guys that are helping me to optimize my builds and learn more about these things. A big thank you!
Byron W5FH