deluxe reverb idele currents zero and 25 but amp sounds good - whats up.

[LoveChet]

One 6V6 measures 25 ma idle current but the other 6V6 measures zero but the amp still sounds good, whats going on ?

My amp was sounding poorly last week and I figured I might have a bad 6v6 tung-sol tube. When I checked the idle currents they were 45 and 50 and I couldn't get them down any lower then 25. So I put in a new set of tung-sol 6V6s, set idle currents to 19,20 and the amp sounded super.

After playing for only a couple hoyrs the amp was still sounding good so I rechecked the idle current since I wanted to bump it up 25 and see how it sounded set a little hotter. What I found was that one 6V6 had wandered up to 25 but the other one was now zero.

Both tubes are glowing.

Any idea whats going on.

[Chuck H]

Possibly bad new tubes and one is already dead. New doesn't always = good. Complaints here of bad tubes coming out of new boxes abound (especially from me ).

Reasons the amp sounds bad/good could be partly due to what volume level you're playing at the time.?. With a hot bias and running the amp at a low level you may not get an unpleasantly asymmetric wave form with only one tube in operation. increasing drive signal should reveal that the push/pull power amp circuit isn't functioning and will sound worse.

Other reasons one tube may not be functioning would include dirty/worn sockets, an open screen resistor or a bad solder joint.

The first thing to do is swap the tubes in their sockets and see if the lack of function follows the tube to the other socket or stays with the socket.

[LoveChet]

Thanks for getting back so quick Chuck.

I'll swap the tubes and check like you suggested.
I have a set of GE 6V6G NOS bottle tubes in the wings but I dont want to use them until I get the amp solid and have some baseline to compare to the NOS tubes.

I agree that tubes can go bad right off the bat. I also have had loose 6V6 sockets which my repair guy fixed but he said I might need new ones.

However, my thought is this. Being 40 years old and probably never upgraded Im thinking of doing an overhaul on the power section so that several variables are out of the equation for the future. What I would change are: tubes sockets, resistors, and signal caps coming from PI. Also possibly couple of things in bias -35 circuit as in doide etc.

I need to run to doctor now but I'll swap the 6V6 tubes when I get back and give you a report. However I'd like to get a complete list from you of what I should upgrade and possibly where to get the stuff. I had the HT power section filter caps changed out 10 years ago.

Thanks for the help
Gary

ps: Working on Chet's 'I Love How you Love Me'

[Chuck H]

If the sockets are already diagnosed as "might need new ones" by the tech that cleaned and snugged them then I would say replacing them as maintenance is a good call. Other than that...

Standard reliability repairs done by techs to these older amps would involve replacing the electrolytic caps (if they're old). Often the electrolytics in the preamp and bias supply are neglected when a power supply "cap job" is done. So make sure to get those too.

I wouldn't recommend arbitrary replacement of film caps and resistors. Extra soldering on those old paper boards has it's own risks. And though film caps can go bad, it's not common enough to replace them as preventative maintenance. Old paper and oil caps maybe, but DR era amps won't have those.

Carbon composition resistors are known to drift out of spec. They're also known to carry some "mojo" so they are used regardless of this even in some modern builds. Standard practice would be to measure resistors throughout the amp and replace any that have drifted out of spec. Keeping in mind that because of how some circuits are arranged some resistors can't be accurately tested in the circuit.

In other words, electrolytic caps and worn sockets, yes. Other stuff that doesn't "wear out" (per se), no. You wouldn't re-bake a cake to make sure it's properly cooked and you shouldn't rebuild an amp that has already been built. Just fix anything that's broken and replace parts that are known to go bad with age. Clean up any non soldered contact point (jacks, pots, plugs, etc.) and that's it.

[pdf64]

...Clean up any non soldered contact point (jacks, pots, plugs, etc.) and that's it.

And, for clarity's sake, to state it explicitly, that includes their 0V common connections to the chassis ground, as well as their regular signal path connections.

[LoveChet]

So for the maint work I should do, i'll do only the 6V6 sockets and and ch2 preamp and bias related electrolytics only, no resistors or no other caps. Since I use only CH2 I assume preamp related electrolytics would mean V2 preamp, and possibly V4 Mix, even V6 PI if there are any there.

I'll do these upgardes regardless, however when swapping the 6V6 tubes , the zero idle current stayed with the socket so I have a issue somewhere around that socket so its not a bad 6V6 tube this time. (Hopefully just the socket) I will need to do some snooping there to find the problem. Im glad a marginal situation finally got worse. Any hints on what might have gone south if its not the socket?

You said no to changing the signal caps coming from PI so Ill leave those alone, unless I find a bad cap during my snooping.

Regarding the pdf64 reply above could you give me a little more specifics on what these additional things I should check since Im new at forum terminology.
as in:
Clean up any non soldered contact point (jacks, pots, plugs, etc.) and that's it.
And, for clarity's sake,to state it explicitly, that includes their 0V common connections to the chassis ground, as well as their regular signal path connections.

And finally since I havnt had the need to solder anything on the fibreboard yet, what would I be dealing with if I had to do that. I assume you dont have to lift the board and I also assume there are no components under the board sandwitched between the upper fibreboard and the lower insulator sheet. In other words I have never had to unsolder or solder anything to a fibreboard eyelet.

Thanks in advance

Gary
77 year old Western mass Chet lover

[Enzo]

Pull the ??? power tube and measure voltages on the socket pins. Pins 3 and 4 BOTH need to have B+. If pin 4 is missing B+, there is your zero current.