1961 Supro 1624T Low Volume Issue. Need to trouble-shoot.

[cluster]

Hello everyone. I'm kind of new here so I apologize in advance if I make some errors in my post. I've gained a lot of insight here from reading these amazing posts. Now I have an issue of my own.

I own 3 1624T Supro Amps which I love. One is from 1957 w/ 6v6 power tubes. One is from 1961 and the other is a 1964 with the OT mounted to the chassis. The 57 and 64 are loud but the 61 isn't and I'm trying find out why. I tried switch tubes but no luck. Next i checked to see if the OT was damaged. The 12" Rola speaker and OT seem original but since the OT was bolted to the speaker and not riveted makes me wonder. The code on the OT reads 570-1 and 166143. I assume the 61 in the second set of digits makes it a 1961 OT which makes sense. I tested the OT and here is what I found. Let me know if this sounds right.

Measured the resistance of each side:

Primary side: 129 ohms across, 61.3 ohms yellow to white centre tap and 69.3 ohms blue to white centre tap
Secondary side: about 0.4 ohms

Checked for open windings: none. both primary and secondary windings tested fine for continuity.

Checked for shorts: measured across the primary/secondary and got infinity which means no shorts.

Checked Impedance using a variac: for every volt on the secondary (i tested a range of 1-10 volts on the secondary), i measured about 35.6 on the primary, which gave me an impedance ratio of 1,267.36:1 multiplied by a 3.2 ohm speaker load gives me a about 4K impedance.

Does this sound right??

I couldn't do the neon bulb test, since I don't have a neon bulb.


Other observations:

Voltage: with the power tubes (6973) out, OT disconnected and a wall voltage of 125VAC, i measured 485VDC coming off the tube rectifier and 475VDC being delivered at the plates.

Can anyone here maybe suggest other things to do/check to find out what might be causing the low volume??

Thanks so much!!

[Jazz P Bass]

Attempt to monitor the Vac reading at the output tube grids.

All controls full up. Inject a 100mv/ 1k signal or bang away on a power chord.

This reading will tell if it is a preamp or a power amp (output tubes, OT) problem.

[EFK]

The 4K impedance is definitely not right for that amp. I think someone replaced the OT with a different Valco OT because for a 1961, it should be the 6003 ink stamped OT which is nominal 8K to a 3.2 ohm load (and some are up to around 9K into 3.2 ohms). 4K is far too low for 6973 under these operating conditions. this all being said, I think you may have something else going on here that is contributing to the anemic output.

[Enzo]

Having the OT impedance off by a factor of 2 ought to be about the same as having the load be off by a factor if 2. And that generally doesn;t result in severely diminished output level.

No neon bulb? Doesn't radio Shack sell them?

Your amp is 50+ years old, my first six guesses might be caps, caps, and more caps. Certainly electrolytics dry out, but paper and film caps get leaky too.

I would apply a signal to the amp input, then trace it through the amp. I would also check the DC voltages at the various tubes, especially if you have a working amp sitting next to it to compare with.

[cluster]

Your right Eric (as usual). Could u recommend a proper replacement??

[cluster]

Attempt to monitor the Vac reading at the output tube grids.

All controls full up. Inject a 100mv/ 1k signal or bang away on a power chord.

This reading will tell if it is a preamp or a power amp (output tubes, OT) problem.

Strumming my Strat at a moderate rate gave me about 5 VAC on the grid (pin6) of the 6973 at the output. Is that about right??

[cluster]

Also, with a wall voltage of 125VAC i measured 380VDC on pin 9 (plate), 362VDC on pin 8 (screens I guess) and 22VDC on pin 7 (which has a 250 ohm ceramic resistor to ground).

[cluster]

Also here is a web link to some gut shots of the amp: My Supro Amps

[cluster]

This OT has a 4 ohm tap. Would it work as a replacement?? Supro S6424 Output Transformer

[EFK]

Your plate voltages are a bit on the high side for one of these, although not crazy high given the 125VAC. Someone has obviously been working in there, some caps as well as replaced piece of shielded cable, probably other work also. Death cap is still hanging off the fuse. And the electrolytic can cap (filters) is original, as noted above, this is probably your biggest problem. That's got to go.

The OT impedance being off isn't going to have a huge effect, but it is noticeable on these. These are not Fenders, I have not found them to particularly accommodating to screwy OT ratios.

The OT you link to is really a tweed deluxe OT, but should work pretty well on the 4 ohm tap. These are true 3.2 ohm speakers, so the reflected back will be a little lower but will be close. Certainly better than 4K which is not making those tubes happy under the operating conditions in this particular amp.

[Jazz P Bass]

Strumming my Strat at a moderate rate gave me about 5 VAC on the grid (pin6) of the 6973 at the output. Is that about right??

5 Vac on the grid will not drive that 6973 very far.
Was that with the controls all the way up?
I would expect at least 25.

You may want to explore why the preamp is so low.

http://www.jogis-roehrenbude.de/Roeh.../6973/6973.pdf

[Enzo]

Again, you have three, and the other two work, right? Open one up and find out what good readings look like.