How do I know what type of rectifier tube my amp should have?

[DLNB]

So I have an old amp made by Lifco, it's a Cobra Mark II (I've posted about it a few times before). Two 6v6's, two 12ax7's. I cannot find a schematic for it but the schematic and layout sheets for a Deluxe reverb look a lot like this amp. Very similar.

When I got the amp it had a 5ugb rectifier tube installed. Very old, almost dead, possibly original? I replaced it with another 5uGB.

Is there any way to know if the amp should have had a 5AR4? I am considering ordering one to see how the amp sounds with it.

The amp sounds good but it runs hot. Not hot as in high temp, but turn it past 4 and your getting into pretty serious overdrive.

I have not biased the amp or measured a lot of voltages. My knowledge is about this is limited but I am enjoying the learning proecess.

[Prairie Dawg]

I'd probably use either a 5Y3, 5R4 or a 5AR4. Both impose less of a load on the 5v filament string and your tube lineup doesn't demand a 5U4GB

[Chuck H]

Most vintage and vintage style amps do start to break up around halfway up with single coils. Much earlier with hot humbuckers. So if your using humbuckers it's not a surprise that your clipping early on the volume control. That amp probably came with a 5y3 stock. I only that because 90% of the tube rectified amps that use a pair of 6V6's also use a 5y3. There's probably no problem with using a 5u4 if the PT isn't getting too hot.

There may be other issues that need addressing with that amp though. If the power supply filters and other electrolytic caps haven't been replaced it's over due. Old resistors could be checked for value drift and bias levels throughout the amp should be checked. Pots, jacks and tube sockets cleaned, etc. If it has a two prong cord it should be changed to a grounded chassis three prong type. If the fuse is on the white insulated AC main it should be changed to the black. I may have forgotten something too.

Does the amp have any problems like excessive hum, sputtering noises, low volume, scratchy pots, squeal when turned up or with a pedal, etc.?.

[DLNB]

Most vintage and vintage style amps do start to break up around halfway up with single coils. Much earlier with hot humbuckers. So if your using humbuckers it's not a surprise that your clipping early on the volume control. That amp probably came with a 5y3 stock. I only that because 90% of the tube rectified amps that use a pair of 6V6's also use a 5y3. There's probably no problem with using a 5u4 if the PT isn't getting too hot.

There may be other issues that need addressing with that amp though. If the power supply filters and other electrolytic caps haven't been replaced it's over due. Old resistors could be checked for value drift and bias levels throughout the amp should be checked. Pots, jacks and tube sockets cleaned, etc. If it has a two prong cord it should be changed to a grounded chassis three prong type. If the fuse is on the white insulated AC main it should be changed to the black. I may have forgotten something too.

Does the amp have any problems like excessive hum, sputtering noises, low volume, scratchy pots, squeal when turned up or with a pedal, etc.?.

Thanks for the reply.

I have already changed every cap. the tubes, and added a 3 prong cord. I have checked some resistors when trouble shooting a squeal issue. Turned it it was a bad resistor to ground. The amp is super quiet, and sounds great. The dynamics are amazing. Play soft and it quiet and mellow, play hard and it roars.
I like the amp a lot the way it is, but I was just wondering if the 5ugb is the correct recitifer and what it might sound like with a 5y3 or 5ar4.

[R.G.]

I'll likely get another load of scorn heaped on me, but why wouldn't you put in a couple of solid state diodes and a power resistor to give you the "fade" you like. You can adjust sag to taste by diddling the resistor value.

(about here we'll hear about slow warm up and cathode stripping, and subtle effects on tone, right?)

[Chuck H]

I'll likely get another load of scorn heaped on me, but why wouldn't you put in a couple of solid state diodes and a power resistor to give you the "fade" you like. You can adjust sag to taste by diddling the resistor value.

(about here we'll hear about slow warm up and cathode stripping, and subtle effects on tone, right?)

Nah. No scorn. It's all good. I won't even design a new circuit with a rectifier tube. I use a resistor for power supply sag instead. One less tube to worry about who's contribution can be emulated by more reliable devices.

Not to turn the thread, but... It's my understanding that cathode stripping in typical audio amplifiers is a myth. A phenomenon that's much more relevant in really high performance, high Vp stuff like tramsmitters. Corrections accepted.

[R.G.]

Actually, cathode stripping below about 700V on B+ is a myth as you say. but it's so prevalently used as a reason not to put silicon in an amp that I expected an automatic reaction saying it. I was lying back in the weeds waiting for "cathode stripping" to raise it's head.

[DLNB]

Most vintage and vintage style amps do start If the fuse is on the white insulated AC main it should be changed to the black. I may have forgotten something too.

This one I didn't do. What is the purpose of this mod?

Thanks for the info.

[Chuck H]

Here:

http://music-electronics-forum.com/t26618/

[DLNB]

Thanks for that.