Extended Foreplay 3 weirdness. Tube rush

[John EH]

I thought I had this worked through but I've noticed after most of a days run the left channel which I'll call tube socket B is causing the tubes to be microphonic and have a considerable amount of tube rush.

I have to confess I'm not working from the appropriate original manual but I do have the Extended Foreplay 3 manual.  Can't find my original manual but I have the manual with the OD3 regulator.

At any rate everything and I mean everything in the Extended Foreplay 3 manual checks out beautifully well within 10% for voltages and dead money on resistance.

The Ext FP manual says the voltages on A and B tubes are the same as the original manual.  I of course don't have the original manual so here's what I have.


A
1 - 83.6
3. - 1.536
4 - 31.41
5 - same
6 - 150.5
7 - 83.2
8 - 83.1
9 - 27.38

B
1 - 93.2
3. - 1.536
4 - 31.47
5 - same
6 - 150.5
7 - 89.2
8 - 89.4
9 - 27.38

So you can see I'm running a little hotter on 1, 7, and 8 on the B channel but a little hotter than 10% on the 1 pin.

Ok here's the oddball thing.  Started getting my tube rush out of the left channel so I shut down.  Ran through the checks, swapped the tubes from side to side and fired up again.  Floor noise in both channels is what I'd call acceptable.  So apparently a cool down did me some good.

What would cause my left channel to run hotter and have tube rush?  This has happened on a couple pairs of tubes. 

All my LED's are glowing wonderfully and steady.  Music is outstanding beyond belief but I'm worried I have a tube killer here.

Q2 on the C4S board?



John

[Paul Birkeland]

That tube that produced the "B" voltages is getting on the edge of acceptability, I'd recommend replacing it fairly soon.

-PB

[John EH]

Thanks. But that's my point.  My FP3 is going through tubes.  I swapped the tubes from side to side.  I guess I should have rechecked the voltages to see what happens.  Can't check it today but tomorrow I'm see what happened when I swapped the tubes. 

It's been on for 8 hours now today with no noticeable tube rush.

John

[Paul Birkeland]

The notion of the higher voltage being "hotter" is a misnomer, the higher plate voltage is a sign of a tube with weaker emission.  This should absolutely follow the tube when you're measuring voltages in your preamp, if you are able to get relably higher plate voltage on one side regardless of tubes, then there would be a strong reason to do some debugging.

-PB

[John EH]

The notion of the higher voltage being "hotter" is a misnomer, the higher plate voltage is a sign of a tube with weaker emission.  This should absolutely follow the tube when you're measuring voltages in your preamp, if you are able to get relably higher plate voltage on one side regardless of tubes, then there would be a strong reason to do some debugging.

-PB

So sounds like my first troubleshooting step will be the right one.  I understand that the higher voltage wouldn't necessarily be "hotter" in a constant current regulator. Poor choice of words.

John

[John EH]

Ok armed with your post I couldn't wait until tomorrow.

The B channel with the A tube is now at 83 volts but oddly enough the B tube (which was marginal) is now at 85 volts on the other side.  Goes for pins 7 and 8 as well.  Weird.

John

[Paul Joppa]

Check your heater voltages (voltage difference, pins 4/5 to pin 9)! From your measurements, they differ by only 4 volts, while is should be 6.3v - the heaters are not hot enough to function properly. Is the preamp getting 120v from the power line?

[John EH]

Check your heater voltages (voltage difference, pins 4/5 to pin 9)! From your measurements, they differ by only 4 volts, while is should be 6.3v - the heaters are not hot enough to function properly. Is the preamp getting 120v from the power line?

I believe you have hit the nail on the head Paul.  I am not getting 120 v.  I live in Japan and get 100 v.  I think I need to go buy a step up transformer. I have one for the refrigerator and the microwave.  Need one for the stereo as well.

I noticed my bias voltage on my ST70 amp was maxed at .85 last night for what I believe is the same reason.  In my configuration on my restoration it should be 1.0.

[Paul Joppa]

Yeah, that's what I thought  :^)  the FP is OK, it doesn't run right but not much harm done. But a power amp - actually anything running near maximum current - running at less than adequate heater voltage will poison the cathodes. Not right away, but way too soon.

[John EH]

Yeah, that's what I thought  :^)  the FP is OK, it doesn't run right but not much harm done. But a power amp - actually anything running near maximum current - running at less than adequate heater voltage will poison the cathodes. Not right away, but way too soon.

Before I was running some cheap Chinese EL34's so I don't care so much but last night I broke out the brown base DD stock Mullard tubes and when I went to bias them they fell short.  I knew I was getting a step up transformer but before your post I wouldn't have thought to add the preamp to it.

Thanks for your answer.  I knew there was something there and I knew one of you guys would see it.  I sure didn't.

[John EH]

Paul,

Just for the record I went out in town and bought a monstrous Japanese step up transformer that took my mains from 103 to 123.

My voltage on pin 4 is 33.5 and my voltage on 9 is 27.3 or about 6.2 volts difference. 

You nailed it for sure.  My bias voltage swung way the heck up on the Dynaco ST70 as I knew it would.  My output tubes are now properly biased.

Everything pretty much sounds the same to these ears but I'm not so worried about whacking expensive NOS tubes now.

Thanks much again.  This place is and always has been the best for this kind of stuff.

To the new builders......take note.  When something is wrong don't be afraid to get in with a meter and list all the pertinent voltages.  All of them if you don't know what is pertinent.   One of these smart guys will see the problem and give you a push in the right direction.

John