Afterglow - now I really mean it!

[pboser]

Hi Paul,
That sounds much easier than finding a place on the chassis!
Fortunately, I spent quite a while on the Mouser site and looking at Ohmite's linked documents and I think i got the right brackets.
I'll work on that while I await the other parts. Progress!
As always, thanks!
Pete

 

[pboser]

I finally got the rest of my parts and got started last night.  I should be able to finish up this evening. Any suggestions on testing before I assume it's ready to go?  I have the Afterglow manual, and could do the testing there, though I assume a few voltages will be different.
I'm champing at the bit!  Thanks again for the help.
Pete

 

[Paul Birkeland]

My only suggestion for testing is to use the cheapest possible 2A3 that you can find!

 

[Paul Joppa]

My biggest uncertainty is the 2A3 voltages, so I'd like to see the DC voltages at each pin of the 2A3 socket.

 

[pboser]

Now you’re making me nervous! I think I’m down to one pair of rca 2a3s. Is there any value in making measurements without the output tubes in?

 

[Paul Birkeland]

So, you could fire the amp with no 2A3, but the driver tube won't work without it.  This would just be something you'd do to be sure the power supply is wired correctly. 

I'd snag a Sovtek or two for testing:
https://www.ebay.com/itm/NIB-new-Sovtek-2A3-vacuumt-tubes-51-each-matched-pairs-or-quad-available/402220779030?hash=item5da63a0616:g:eYgAAOSw1tVek5Xy

 

[pboser]

I looked around the house and found an Electro Harmonix (Sovtek, right?) 300B and a pair of Sovtek 6B4G, but no other 2A3s, so I think I'll take your advice to be cautious and get some new 2A3s for testing purposes.  More waiting to hear the amp, but it seems smart to play it safe. Thank you gents.

 

[pboser]

Got the tubes - NOS Arcturus 5965s and new Sovtek 2a3s.  Plugged them in and here's what I got for voltages on the output tubes:

Pin 1- 189/219
Pin 2 - 452/443
Pin 3 - 139/178
Pin 4 - 189/221

So, what gives?

As always, thanks in advance for your support and help.
Pete

 

[pboser]

After sending the note above I had the idea to swap the tubes and see what happened.  I swapped both the 5965s and the 2a3s.
When I did so and turned the amps on, the second one had no audio!  Here are the voltages:

220/102
442/479
179/19(!)
220/102

What did I do?!?!

 

[Paul Birkeland]

Pin 1- 189/219
Pin 2 - 452/443
Pin 3 - 139/178
Pin 4 - 189/221

What do the slashes mean?  Is that 189 on one amp and 219 on the other?

 

[pboser]

Yes, that’s right.

 

[Paul Joppa]

Well, you want 2A3 pins 1 and 4 to be about 200v, pin 3 about 155v. One of the 5965s seems to get you there in either amp - I'll tentatively conclude that the amps are basically wired correctly. The voltage is a little high (220 not 200) but that might go down with some tens of hours burn-in, so I won't suggest optimizing it just yet.

But the seemingly bad 5965 is inconsistent - it works differently in each amp. Could be oxidized tube pins of course. Or a difference in the tube/socket interface, like worn socket contacts, not-quite-straight tube pins, etc. Check to be sure that small movements of the socket contacts can't cause unwanted shorts.

 

[Doc B]

But the seemingly bad 5965 is inconsistent - it works differently in each amp.

Maybe the two amps are wired to swap triodes, so one amp is uses the left triode for the voltage amp, the other amp uses the right triode.

 

[pboser]

Hi Doc,
I remember that Buddha suggested that setup for the 5965, but I'm pretty sure the triodes are paralleled.  I checked the assembly manual and it seems I haven't changed that part since the beginning.

PJ,
I took out the small tubes, cleaned the pins with scotchbrite pad, straightened the pins, and looked at the underside of the socket.  I might have had the new cathode resistor too close to the adjacent pin, so I bent it away.  Put the tubes back in where they had been most recently - the second test.  Here's what I got:
220/190
441/450
173/139
220/189

So, on the theory that one of my new NOS tubes may be ng, I dug out another pair of 5965 - not the ones that were originally in it a few weeks ago.  And I got this:
203/207
445/445
159/164
203/207

So that's pretty good, right?  What do you think about the pin 2 readings?

 

[Paul Joppa]

Pin 2 is just where I hoped it would be  :^)

Those voltages look satisfactory to me. Listen to it for a few weeks to be sure the 5965s have settled into their groove, then re-check the 2A3 voltages - then we can talk about refining the voltages if it seems worthwhile.

I expect it will sound different now ...

 

[pboser]

Okay, I'm ready to find out!  Thank you, PJ, PB and Doc!