Homebrew 5F6-A, lots of gain

[Bob M.]

I just finished a homebrew amp based on a Bassman 5F6-A circuit. The amp really screams, has a great rock n' roll sound but doesn't have alot of clean before the fun begins. I rarely get a chance to do that loud rock thing anymore so I think I have to tame this beast down a bit so it's more useable for me. It's a stock 5F6-A circuit except 1) ss rectifier, 2) I'm running the screens from ultralinear taps, 3) I'm only using one volume control and one input (I've got the bright on a pull switch pot) and 4) I'm running the first preamp tube (V1) with both triode sections in parallel. It's in a small cab with 1x 12" speaker and the chassis is also very small; the whole thing being about a third of the size of an original 5F6-A amp. I love the sound but after '2' on the volume, it's a shredders dream, lots of distortion so that amps doesn't have the 4 or 5 numbers of clean that most 5F6-As will normally have. The UL taps are because I had this transformer around and I thought I'd try it. Also, V1 in parallel is an experiment and that where I'm wondering if that's where all the extra gain is coming from? Additionally, I have a switch on V2 so I can leave the cathode unbypassed (stock) or add the appropriate cap in parallel. When switched in, it adds fullness but not much extra gain. I have a slope control (47K~100K) and a choice of tone stack caps (,047 and.1 or .022 and.022) but these are features I've used on many amps in the past with success and I don't believe these are the source of my gain problems. The rest is straightforward. The only thing that seems unusual voltage-wise is on grids (8.5 Vdc) and cathode (13.5 Vdc) of the PI. These voltages seem low but it could result from feeding the PI just after the UL taps in the power supply. Any insights would be helpful.

Thanks,
Bob M.

[MWJB]

"These voltages seem low but it could result from feeding the PI just after the UL taps in the power supply." Your PI supply should be coming from a dropping resistor, after the B+ supply, don't your screen supplies come from taps on the OT? Not sure what you mean here?

What are your plate voltages for V1, v2 & V3? Tone stack is fed by a cathode follower as per 5F6A?

Yes, the parallel triodes of V1 will bump up the gain somewhat. I assume that you have halved plate resistor & cathode resistor values, compared to a single triode?

[Bob M.]

MWJB, Thanks for your reply. The audio circuit is exact to the 5F6-A, with the cathode follower driving the tone stack as per the schematic. Most of my changes are side issues except for V1, dual triodes in parallel. My voltages are as follows:

B+ : 450 Vdc
Plate : 441 Vdc
Sc : 442 Vdc
PI : 406 Vdc
V2 supply : 347 Vdc
V1 supply : 345 Vdc - I have decoupled V1 from V2 via a 4.7K resistor and a 22uf/350V cap.

V1 voltages:
Pins 1,6/Plate : 238 Vdc
pins 3,8/cathode : 1.73 Vdc

V2 voltages:
Pin1/plate : 204 Vdc
pin3/cathode : 1.24 Vdc
pin 6/plate : 347 Vdc
pin 7/grid : 204 Vdc
pin 8/cathode : 206 Vdc

V3 (PI) voltages:
Pin 1/plate : 368 Vdc
Pin 2/grid : 8.67 Vdc
Pins 3,8/cathode : 13.84 Vdc
Pin 6/plate : 365 Vdc
Pin 7/grid : 8.70 Vdc

After the UL transformer, the filter dropping resistor to the PI is a 6.8K@7 watt but this is experimental and I'll eventually use a 10 watt unit when I decide on the proper feed voltage for the next stage. Next is a 10k/1 watt to V2 and, as stated above, a 4.7K/ 1/2 watt to V1. The grid and cathode voltages for the PI - V3 seem low compared to my other Bassman amps (I have two others). I'll try another tube/set of tubes to determine if that's a factor.

Also, I plan to try a 12AY7 in the V1 position. I had been using a 12AX7A at V1 as most players do. I also though about making the dual parallel triodes of V1 a switchable feature as it does sound very good in parallel but not very clean. As my amp's a homebrew, it wouldn't be a hardship to do this. I can do with a 3PDT or 4PDT switch and have both sounds/wiring methods. I will also try disconnecting the parallel triode at V1 and running it stock to compare before adding a switch. Yes, I have halved the plate load resistor (47K/1 watt) and the cathode resistor (750R) for the parallel mode, which are about half of the normal stock values.

I'm also going to try raising the value of the 270K summing resistor post volume control, into V2, to about 470K or thereabouts. Part of my situation in not getting enough clean sound is that my power transformer is smaller than the stock 5F6-A power transformer. The spec for the power transformer that I'm using is 325-0-325@150 mA. Any additional comments /ideas would be helpful.

Thanks,
Bob M.

[loudthud]

Substituting 12AY7's and 12AT7's is an easy ways to reduce gain.

One thing you didn't mention is the feedback resistor and the impedance taps in the power amp. Many people don't use any feedback at all with UL taps and that gives about the same or slightly higher gain than the standard 5F6A. You might experiment with feedback resistors as a way of reducing gain.

[Bob M.]

Thanks, Loudthud, I will try experimenting with the negative feedback circuit. Currently, I'm using the stock 27K feedback resistor in the normal configuration. My output transformer has 8 ohm and 4 ohm taps; I'm using the 8 ohm tap with a single 12" speaker. I was initially going to use a EVM 12-L speaker but with this particular amp and circuit, I've revised my thinking, after listening tests, in favor of a 8 ohm 12" G12 Vintage 30 Celestion speaker.

However, I'm happy to report that I found the low voltage error in the PI circuit: builder error - I got the cathode resistor wrong. I put in a 4.7K where a 470R is called for. Once replaced, the correct voltages magically re-appeared. It's funny how that works out. Using a 12AY7 in position V1 gives a plate voltage of about 75 volts less than the 12AX7 in that position. Right now, it's a bit too late at night to crank out but tomorrow I will and I expect all to be vastly improved. There's always a reason for these things and this time is no different. With a brand-new amp there is always something that needs correcting but I need to be better at finding and correcting my own mistakes (at least the amp powered up, with sound, on the very first time!!) Thanks for all your comments and insight.

Bob M.

[gwbdopesmoker]

I just started working on a UltraLinear amp
(converting a Carillon 75) and one thing
I noticed is the hum is horrible.

I have 11 vAC RMS ripple on the plates when the
amp is idle at 24watts ( 2x EL34).

This is a piece part amp you put together ?

Gain is lost in the tone circuit .. but then picked
up again by a voltage amp before the phase splitter.