Alamo Paragone Special Tube Amp - Help!!

[gtrplr2]

I am working on an Alamo amp and am out of things I know to check. I can not find a schematic or any thing on this amp. All tubes test good on my Hickok. The tube configuration is 1-5U4GB rectifier, 2-5881 power tubes, 2-12AX7A preamp, and 1-7199 inverter. I have 120VAC on the transformer primary, 395VAC on pins 4 and 6 of the rectifier (measured from centre tap to each pin) 6.2VAC on the green heater filament secondary and 5.0VAC on the yellow rectifier heater secondary. I have 520VDC on pin 2 of the rectifier when one test lead is hooked to ground and the other test lead hooked to pin 2. I have 520VDC on the plates of each 5881 (pin 3) I have 0VDC (Zero) on one 5881 and +120VDC on the other 5881 on pin 5 where I should see -bias voltage. The transformer secondary centre tap is hooked to the standby switch on one leg with the other leg going to ground. I have replaced the power supply caps and the electrolytic caps and checked every single component in the amp, all testing good. I also replaced the multi-section aluminum capacitor can (20uf-20uf-20uf)/450V with three 22uf/450V individual caps inside the chassis. There are no resistors on the cathodes. I have no idea how this amp develops its negative bias supply or why my readings are so off on the power tubes. Any help would be very much appreciated.

[hasserl]

I've got a Paragon Bass, my voltage are much lower than yours, I'm seeing B+ of 422.

You've probably got a leaky coupling cap from the phase inverter to the 5881 w/ 120vdc on the grid. That is a .02uf cap.

There should be a 400ohm resistor on the power tubes cathodes, w/ 50uf bypass cap.

Note the amp uses ultra linear output trans, and grid leak bias for the input stages.

[Enzo]

Pull the power tubes out. If that +120v on pun 5 goes away, then the tube was internally shorted. If the +120v remains on pin 5 of the socket, then look for the cap going back to the prior tube - the phase inverter tube. Replace it. Might as well do the one to the other pin 5 while you are in there.

What is the voltage on pins 8 of the power tubes? Or with power off, what resistance to chassis is there for pins 8? Unless the wires from pins 8 go directly to a ground lug right at the socket, follow any wire from either one to see where it goes. I bet the two are connected together, but I also bet there is a wire trailing off to an electrolytic somewhere. You could have a cathode resistor way across the chassis.

You also didn't mention screen voltages.

If they are grounded - the cathodes, that is - then there must be a negative supply. And a common reason for those to go away is installing a filter cap backwards. We get used to installing them with negative to ground. But the exception to that is the bias supply with positive to ground. If there is a negative supply, there will be a rectifier for it.

[Enzo]

Aha! Simulposts.

[gtrplr2]

Thanks for the replies Enzo and hasserl! I will try to answer all of your questions at one time. All of my readings were taken with just the rectifier tube installed. I first cranked it back up initially and checked the bias and noticed the bad bias and shut it off right away, and pulled all the tubes and started looking around. There is an electrolytic attached right at pin 8 on the first 5881 directly after the power supply cap (negative end to ground and positive end to tube pin). I replaced it like the old one that came out with the positive end hooked to pin 8. I know what you mean about the positive to ground on the fender amps. There is no voltage on pin 8 with the power on, standby switch in the on position. There is also no resistance to ground with the power off. I have the 400 ohm wire wound sand resistor on the second 5881 (wired to pins 8 and 1), and in turn is hooked up to the first 5881 pin 8. The screen voltages on both 5881 tubes are 525VDC. I will also replace the .02 resistor between the inverter and the 5881, they check good with my DMM, but my ESR is broken and I had to order a new one, Thanks for all your help!!!

[gtrplr2]

It works!!

Well I changed out the inverter .02 bypass cap, and now it is where I can fix the rest of the stuff. I now have 435VDC on the plates, (all tubes in and driving a 2-12cab) and all other readings are back to normal. I did change out the other .02 cap that was right there along with the 470K resistors. I really want to thank you two (Enzo and hasserl) for all the helpful advice! I just joined this forum and have posted this very question on other forums that I have been a member for years, and this one is the only one to answer back. Thanks again!!

[hasserl]

You're welcome. Bitchin sounding amp, ain't it?

[gtrplr2]

Yes it is, It is a great souding amp! I am actually surprised! It is a friends of mine that just sent me the chassis from out of state, so I have it hooked up to my 65' Bassman cab. I guess the original cab had a 15" speaker? Thanks again!

[hasserl]

Yes, a Jensen C15N, in a bass reflex front ported cab.

I've got quite a few amps around here, like a Fender Princeton Reverb, Bassman and Super Reverb, and a Weber 5E5A. And I work on a lot of other amps, Ampegs, Marshalls, Carvins, Peaveys, you know, the usual. The Alamo compares favorably to any amp. They are all different, and I hate saying one is better than the other, but the tone of the Alamo is just as dynamic and harmonic laden and tonefull as any. And mine is still in original condition, badly in need of new caps and a three prong cord. It sounds friggin great anyway.

[gtrplr2]

Well now that it is where I can actually put the tubes back in and play, I am probably going to have to change out some filter caps. The bass channel works great on all the tone pots, but the normal channel bass control does nothing at all. I have cleaned all the pots completely with deoxit with no luck. Both channel volume pots are scratchy when rotated, ground? I found all the input jacks open and that really helped with the hum! This amp has been a real project for sure.

[Enzo]

Scratchy pots that don't clean? Look for DC across the pot, likely from a leaky interstage cap.

[gtrplr2]

Thanks again Enzo,

I have been out of town and had not had a chance to reply. Back to the Alamo! I have a CTS pot that measures 500K. It is a CTS 12th week of 65(volume pot) but I am not sure of the other markings. Does "L" mean it would be a linear taper instead of an audio taper? Here is the complete number:
ML5619 1376512