Paramount eats tubes

[Aural Robert]

My left channel started fading away, so I switched the 2A3 tube with the right amp. The fade followed the tube so I assumed a bad tube. Put another set in, it's all good.  Then about a week later, my left channel fades away again (maybe to 1/2 volume or so). I'm thinking, another bad tube? I don't think so.

I ran all the resistances and everything checks out. I don't want to do a voltage test for fear of damaging yet another tube. So, I am seeking knowledge and advice as to where to go next.

TIA. Aural Robert.
... neurosurgeons scream for more ...

 

[Paul Joppa]

That's not enough information to draw conclusions, so here are some questions to get more information:

Presumably, this is a 2A3 version (i.e. direct coupled)?

Do you have the "soft-start" mod installed or not?

Did you swap the 2A3s the second time it happened?

Are the new tubes new production or new old stock or used old stock?

Why do you think checking voltages will cause damage?

 

[Aural Robert]

Hi Paul. Yes, these are the 2A3 version.

This is a V1.1, the cover page of the manual says "with soft start".

No I didn't swap the tubes the second time. The amp ran fine for months, and I had upgraded to a set of JJ 2A3's. Once the first JJ showed degradation, I put in the original pair of Sovteks and the degradation occurred again on the same amp.

All the tubes have been new production - JJ and Sovtek.

Having burned out a premium tube or maybe 2 of them, I'm being cautious. I am totally illiterate when it comes to the theory of this stuff, so I don't want to risk causing more damage by forging on without knowing what I'm doing. Thus my posting.

Aural Robert.
... freak out in a moonage daydream ...

 

[borism]

Hi Robert,
While I only have a 2A3 Stereomour I had a similar experience with two pairs of the JJ 2A3-40. Both times one of the tubes started getting softer (fading away) and consequently the center image started shifting to the strong tube side. With one pair this happened very early and the other just short of 6 months.
Nothing was/is wrong with the amplifier.
My suggestion, as long as the voltage measures check out (with a good tube) on the offending side (? left), is to use one of the damaged tube pairs with the strong tube on the left side and see if the same problem repeats on that side. If not, it's not the amplifier but coincidence and failing tubes.
Good luck!
Boris

 

[Paul Birkeland]

You'll want to do voltage measurements, with the additional measurement of DC voltage between pins 1 and 4 on the 4 pin socket with a 2A3 installed.

 

[Aural Robert]

Hmm, so it turns out I'm blowing fuses now. I will triple check my wiring and report back.

Aural Robert
... she's as beautiful as a foot ...

 

[Aural Robert]

OK I've finally been able to get under the hood. All of my wiring is ok, all of my resistances check out. I put in a new fuse, turned it on and it blew again.

Very strange as this amp was working for a few months at least. Then it ate 2 different 2A3's and now it won't power up at all. Do I have a transformer problem or something?

HELP!!!!!

Aural Robert
... puttin' on the ritz ...

 

[Paul Joppa]

This is the problem with direct coupled designs - it's too easy for the output tube to be wrongly biased.

It's possible that the problem is with the driver tube.  Swap them between channels, cross your fingers, and see if the fuse holds.

 

[Aural Robert]

OK, I swapped the small tube from the other amp and the fuse blew almost immediately again.

I am running GE JAN-5670W & Sovtek 2A3 if that helps.

Aural Robert
... And I'm just about to lose my mind ...

 

[Paul Joppa]

OK, I guess we're not going to get lucky.  :^)

Check the 220 ohm grid stopper resistor at pin 3 of the 2A3 - just measure the resistance across it. That resistor is fragile, and if it's blown you would likely see too much current in the 2A3, thus blowing the fuse.

PB has the most experience with diagnosing Paramount problems, so hopefully he will chime in at some point. Meanwhile, try this: remove the 2A3 and see if the fuse holds.

Assuming it does hold, check the voltage at OA (300v) and OB (175v). Don't leave it on any longer than needed - without the 2A3 the board sees too much power supply voltage and will overheat. This will tell us if the board is working properly.

 

[Aural Robert]

The resistor reads 239.9 ohms so we are within tolerance there. I removed the 2A3, put in a new fuse and poof it blew immediately again.

I'll pull up the board and make sure everything is well attached. I did check all my wiring and continuity and it was all good.

Perhaps one of the resistors on the board? I'll check them all out too.

 

[Aural Robert]

I touched up all the solder pads on the board and ensured everything is connected to the correct terminals etc. All the resistors are within spec.

I'm down to my last fuse at the moment so before I power up just want to check if I should put the 2A3 back in? Anything else I should look at?

Aural Robert.
... If you take a walk, I'll tax your feet ...

 

[Paul Joppa]

My best hope was that the 2A3 was drawing excessive current; apparently that is not the problem.

If you haven't, repeat the resistance checks in the hope that is will reveal something. Stop there and post if you find a problem. If not, do this:

At the bottom of page 18, there is a picture of the PSU board, showing the high voltage output of the power supply - the twisted pair of black and red wires. Disconnect the red wire from the board temporarily - this will isolate the rest of the circuit from the power supply, so we can determine whether the problem is on the PSU board or in the rest of the circuit. Without the 2A3s installed, fire it up to see if the fuse blows. If it doesn't, repeat with 2A3s installed. If it still does not blow, do the voltage checks.

If the fuse still blows, get some 1.5 amp fuses - sometimes a different manufacturer will affect the fuse performance.

(I'm doing my best here to avoid messing with the PSU board, which is very difficult to remove from the power transformer without damage.)

 

[Aural Robert]

Here are my resistances:

T1 manual reads * I get *
T2 manual reads Nil I get 0L
T3 manual reads Nil I get 0
T4 manual reads Nil I get 0L
T5 manual reads 4K I get 4.00K
T6 manual reads 4K I get 4.12K
T7 manual reads * I get 269.7K
T8 manual reads Nil I get 0
T9 manual reads 124K I get 269.7K
T10 manual reads 0 I get 0
T11 manual reads 3.0M I get 0L (my meter does this >1M)
T12 manual reads 5.6M I get 0L
T13 manual reads 0 I get 0
T14 manual reads 9.2M I get 0L
T15 manual reads 15M I get 0L
T16 manual reads * I get *
T17 manual reads 0 I get 0
T18 manual reads 0 I get 0
T19 manual reads 128K I get 269.6K
T20 manual reads 249K I get 248K
A1 manual reads 4K I get 4.02K
A2 manual reads * I get *
A3 manual reads * I get 270.2K
A4 manual reads 4K I get 4.01K
B1 manual reads 0 I get 0
B2 manual reads 8.2K I get 5.85K
B3 manual reads 249K I get 248.3K
B4 manual reads 130K I get 269.4K
B5 manual reads 0 I get 0
B6 manual reads 177K I get 269.7K
B7 manual reads 220K I get 243.3K
B8 manual reads * I get *
B9 manual reads * I get *

Some of the resistances are double the manual so I'm not sure if that's ok?

Aural Robert
... Searching high and low ...

 

[Aural Robert]

Hi guys, any advice? Am I ok to move to the power up stage and test the PSU board as Paul has recommended?

Thanks.

 

[Paul Joppa]

I think those are acceptable. The 8.2K depends on the setting of the trimmer on the board, so you can expect some variation. I have a dim memory of the 130K/250K error but I can't find the reference. However, since it worked before, I think it's safe to fire it up and check the voltages - keeping a weather eye for smoke!

 

[Aural Robert]

Looking much better. It blew the 1A, so I put in a 1.5A and it held. Then I put the 2A3 in and it held, and all the tubes light up.

I ran my voltages and most are within 5% or so, except for A3 - the manual reads 217 and I get 151 (152.3 with my other 2A3).

Also Kreg A is supposed to be 4.1 and I read 2.93, Kreg B should be 6.3 and I get 7.07.

So how is this looking? Thanks.

Aural Robert.

 

[Paul Joppa]

That's way better than I feared!

Don't worry about the kreg voltages, they are pretty variable and those are within a reasonable range.

A3 should be 1/3 of A2 (the manual is incorrect); you are probably close to that, and if not that's why there is a trimpot on the PC board. See the manual for details.