Standby and Bias questions of first amp build

[jonnyeye]

I've been looking into building my first amp for a while now, and finally settled on the Super Single-Ended design over at Angela. However, frugality means that I have to substitute the 6V6s (which I do not have) for 6P1P-EVs (which I do have, in quantity; they are Russian tubes, a lot like 6AQ5s). I also purchased smaller power and output transformers than originally specified (the lower voltage should result in less power output, so I think I'm okay), and I will be using a solid-state rectifier, and making a couple other changes (see the attached schematic). Voltages are approximately A: 265V, B: 255V, C: 254V.

Here are a few questions.

1. Is this a good arrangement for the standby switch? The switches I have available are rated for 250VAC (which I can guess is not the same as 250VDC), are they suitable?

2. The 6P1P tube has two pins which are connected to the plate. This allows one to connect a bias resistor to one of the pins on each tube. When both tubes are in place, the resistors are paralleled (giving you a bias resistor of 1/2 Rb); but when one tube is removed, the remaining tube only sees one resistor, giving you a bias resistor of Rb. This allows me to pull one of the output tubes (to halve the power, for playing in smaller rooms) without doing any other fiddling. That said, what is the best choice of Rb? The original schematic called for 250 ohms (so two paralleled 500 ohm resistors), but using the values given for a 6AQ5 in the RCA tube manual (-12.5V bias, 45ma) gives 135 ohm (two paralleled 270 ohm resistors).

3. I have a concern with putting 90ma through the output transformer (the Hammond 125J), although a similar design is being sold with this exact transformer elsewhere. Also, given the transformer I'm using, is the choice of output impedance the better choice for single- and double-tube operation (the transformer has 2.5k and 4k taps)?

Any other thoughts/ideas/input is appreciated.

[Regis]

You don't need a standby with an amp that small.

[Enzo]

While that is a reasonable place to insert a standby switch, I agree that a standby switch on an amp like this is pointless.

You do realize that reducing power by half only reduces volume by 3 decibels.

[jonnyeye]

While that is a reasonable place to insert a standby switch, I agree that a standby switch on an amp like this is pointless.

Okay, it's gone. Saves me a few bucks and a bit of hassle.

You do realize that reducing power by half only reduces volume by 3 decibels.

Yes. But both configurations are running into the output transformer at the same impedance. Theoretically the single tube will distort sooner (giving more crunch at reduced volume), while the double tubes will stay somewhat cleaner longer, for more headroom. That's the plan, anyway. Given the size of rooms I usually play (small), and the fact that I need both clean and dirty, but not at the same time (I'll might run trumpet with effects through the thing one day, then the next go to a blues jam), I thought it might be a simple way to add a bit of versatility.

[Enzo]

Oh by all means go for it. I just note that a lot of folks think that cutting power in half will cut the volume in half as well, and it won't. Just wanted to make sure you were not one of them.

[Ptron]

Wouldn't it be possible to install a 1/2 power switch that cut the cathode to one of the tubes?

[Regis]

A couple of other things you could do:

Put a Pentode/Triode switch in it. I just put one in my Champ I built and it cuts the volume quite a bit.

Build a small attenuator like my Ampwell House. You wouldn't need to put it in a coffee can, just use an ordinary enclosure. You probably wouldn't need a fan with an amp that small.

In fact, you could build a simple and cheap 6 db attenuator with a few resistors from Ted Weber, see the attached pic. That is one I drew up for a 100 watt 4 ohm attenuator but you could use just one 50 watt resistor on each side and that would be ok for the amp you are building. The resistors are only a couple bucks each. If you wanted it 8 ohms just change the resistors accordingly.

[loudthud]

R1 seems to me to be on the high side. I'd start with something like 10ohm/1W to 47ohm/2W. R2 seems to be on the low side, I'd start with a 10K. For the power tube cathode resistors, the pair of 270's sounds like a good starting point but you'll want to experiment to get the best performance out of the tubes you are using. The circuit around the input jacks could us a little work, check any Fender blackface schematic.

[jonnyeye]

Thanks to the people who have responded so far!

Wouldn't it be possible to install a 1/2 power switch that cut the cathode to one of the tubes?

Sure, as long as you were sure to double the value of the cathode resistor at the same time. Still, I don't imagine that I'll be wanting to use this on stage, so tube pulling is fine for me.

Put a Pentode/Triode switch in it. I just put one in my Champ I built and it cuts the volume quite a bit.

This is tying the screen to the plate (possibly through a resistor), right? I'll have to look into this - seems simple enough.

In fact, you could build a simple and cheap 6 db attenuator with a few resistors from Ted Weber, see the attached pic. That is one I drew up for a 100 watt 4 ohm attenuator but you could use just one 50 watt resistor on each side and that would be ok for the amp you are building. The resistors are only a couple bucks each. If you wanted it 8 ohms just change the resistors accordingly.

I'll remember this is it's still too loud after the other mods!

R1 seems to me to be on the high side. I'd start with something like 10ohm/1W to 47ohm/2W. R2 seems to be on the low side, I'd start with a 10K. For the power tube cathode resistors, the pair of 270's sounds like a good starting point but you'll want to experiment to get the best performance out of the tubes you are using. The circuit around the input jacks could us a little work, check any Fender blackface schematic.

I chose the power supply resistor values based on the max ratings of the tubes (Russian tubes don't like overvoltages much, so A is about 270V, B is 255V), with the consideration that the B+ on the preamp stages in the original design was about 260V (C is about 250V, close enough). I have a few other resistors (of smaller value) on hand to sub in for R1 if I find it doesn't get well driven when dimed, and the Russian tubes are cheap enough that I can afford to burn through a few experimenting.
I figured I'd have to experiment with the bias resistors, but I was hoping someone out there would say "this is it!" and I could avoid that small hassle. Oh well.
As for the input arrangement, I've never understood exactly what's going on in those blackface schematics, so I just took the single input setup from the original schematic and doubled it (doubling the value of the grid stopper resistor as well, to compensate). I might try it both ways and see if the extra resistor makes any difference.

One thing that I've just noticed is that the fuse on my schematic is too small. What value should it be? 1A slow-blo?

[loudthud]

For a line fuse, the transformer is rated at 45VA so 0.5A is about right. You have a lot of voltage to drop so that R1 will get pretty hot. Leave enough room for a 10W resistor with plenty of space for heat. Speaking of which, when you pull one tube, the B+ is going to jump quite a bit. Something that may cause problems with those tubes.

Blackface inputs do a couple of things. If input 1 is used by itself, you get a 1meg to ground, and the two 68K are in parallel as a grid "stopper". If input 2 is used by itself, you get the two 68K's in series to ground and the tube grid is connected to the junction of the two resistors giving you 6dB of attenuation. When both inputs are used, the two 68K's act like mixer resistors to isolate the two input sources. The way you have it drawn, you get 6dB attenuation from either input and the 1megs don't do very much.

[black_labb]

You don't need a standby with an amp that small.

being solidstate rectification, woulnt it save the tubes a fair bit?

[Enzo]

No, cathode stripping and other phenomena pertain to large high voltage, high power tubes. It is an issue to transmitting tubes in radio stations, but not to something tiny like this little guitar amp. That is why the amp never came with a standby.

ANy stress on the tube is minimal. it will wear out from use long before anything the standby would have done for it comes along.