Need help with decisions for PA to Guitar amp convo

[MelodianChew]

Hi, K,

Thanks for the reply. I really appreciate it. Have just a couple questions in reply:

The "really first thing" is to build a Power Limting Safety Socket for your test bench

I actually have built one of those just for this build. I will also be purchasing a variac to be able to ease into the older components of unknown condition.

In any case, you need a load resistor that can briefly withstand the full output of the amp, but first, you do a test with an open load. 

I plan on building a heavy duty high watt load resistor at 8 Ohms and wired to a 1/4" phono plug for ease of connection to amps I build.

You mention doing a test with an open load. Is this for the PT and OT tests specifically? Also, Aren't we supposed to NEVER run a tube amp without a load? What is running an open load test going to tell me, briefly?

There are posts about how to do power tests in the Test and Measurement section of this forum,

Thank you for this, I shall see.

The power amp is similar to many tweed-era PAs and will have a warm basic tone.

Which Tweed model or models is my little amp most similar to, or which specifically did you have in mind when you made this statement, because it would be nice to download a fender schematic that I could sit next to mine to be able to compare and to use as a reference for mod and inspiration and to better help me see the differences between the PA and the guitar of the circuit.

The next mod is to add cathode-current sense resistors and meter jacks for safe idle measurements. We offer this as BMK2 Bias Mod Kit for two tubes, or you can buy the parts from any broadline electronics distributor. TUT3 shows how this is wired.

Considering all of the help I am getting from the use of this site, it only makes sense for me to reciprocate with checking out a few of your wares, especially the TUT3 and bias mod kits.

The plate filter cap is woefully undersized and should be 100uF minimum.

Which cap type should I use for this replacement. Do you have any suggestions about which type of composition should I use for the caps and resistors you mentioned for the mods you proposed?

The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.

Well, I have the most unfortunate and unoriginal obsession with the clarity, articulation and singing dynamic gain that is heard in Hendrix's live recordings, the Winterland Concerts specifically. Its just sublime and I can't help myself. And I also play single coils almost exclusively. So maybe that helps.

You could also simply lift one end of R23 to hear the open-loop sound and gain from the PA. If you like that sound, you could add a switch to open/close the loop, or add a pot to dial between 5k6 and nearly open-loop.

Wow. Great idea, thanks.

The mic transformer sockets could be used as preamp tube sockets if you want to try out octal-based types. generally, they sound a bit mellower than the equivalent-mu subminiatures. As TUT suggested, you can achieve a similar sound using parallel triodes without gain loss, or use lower-mu tubes with the attendant gain reduction.

OK. bypass themBut suppose if I were to want to just keep it simple and not use the trans sockets at all, how would I go about wiring their elimination? As it stands, I would have to  to move the inputs and bypass them obviously, right? So I should lift the wires that go from the input jacks to the trans sockets and move them to V1 and V2, respectively, with observation of proper impedence matching, correct?

If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

Hmm, makes sense. What voltage and current PT should I look for? Center tap? Any ideas for model suggestions? Of course you may not be able to answer this besause we arent sure which tubes, right? When you say octal tubes you mean like the 6QN7 (i think that is the number) type or similar?  Or do you have any expert saggestions? I think I actually have some NOS GE's somewhere. Wow this is interesting.

Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

This is because they will be part of the preamp and not the power amp section right?

Could one of those trans sockets work as a gain stage for reverb? I have a cool old tank reverb..


Thanks for all of the help.

Scott