Gibson Ranger GA20-T Rebuild

[guitician]

One thing about that "Death Cap", it may be needed, if later you notice that there is a static electricity type of pops when touching the strings. I have an amp that uses a power transformer that does not have a electrostatc shield around the core and it will do this without a 250V ac cap on the primary. I dont know if that MM replacement uses a copper wrap around their cores.

Edit: just noticed post #7 so your ok without the cap.

[TomCarlos]

Gents... Parts have arrived and I'll work on the amp this weekend.

A question on the Cap across the 200 ohm Cathode resistor for the 6V6 tubes. The schematic shows a 20uf cap. The current part is an electrolytic, looks like a 25 watt rating. Is this a place where you can use a Non-Polarized cap? I've got a 22uf, 100v Non-Polarized cap that I was going to use here. Just curious if this would work here (or cause some odd problem).

[Tom Phillips]

...Is this a place where you can use a Non-Polarized cap? I've got a 22uf, 100v Non-Polarized cap that I was going to use here. Just curious if this would work here (or cause some odd problem...

That will work just fine if you can fit it in physically.

...The current part is an electrolytic, looks like a 25 watt rating.

Note: That's a 25 "Volt" rating. WVDC stands for "Working Volts Direct Current."

[TomCarlos]

Thanks TP !!

[guitician]

That looks like one of those fusible resistors covered with sand. I guess so that if it were to be overloaded it wouldn't catch fire, just burn open.

[soundmasterg]

Here are a couple pics of the amp I overhauled for my uncle. Sorry for the poor picture quality...I took the pics 10 years ago with an early digital camera that had trouble with close up pics. The amp didn't work well when he gave it to me to fix...a very loud hum at all volumes and lots of crackles and pops. Two power supply electrolytics had leaked physically in the amp chassis, and every single cap except the domino mica were bad. This one is a 1958 GA-20T btw with original Jensen P12R speaker.

Greg

[TomCarlos]

Thanks for the pics Greg....

I finished installing the parts on this amp and did a slow power up. So far, the dc voltages look ok. Good thing I had an extra Rectifier tube! The original couldn't produce the expected dc on the Cathode (350vdc).

Of the two channels, Channel 1 (Mic or Instrument) is working the best. But I don't think I'm getting the output that I should. I'm working with old pre-amp tubes and need to convince the owner we should swap those out.

Channel 2 has some issues... there is some oscillating and other noise when I turn up the volume. I'll deal with that later.

For now, I want to work on Channel 1 - replace 12AY7, 12Ax7, and drop in some new 6V6 tubes.

As for the output transformer, I have read in other threads that the OTs on the older Gibson's are problematic. I'm wondering if I could use something like a Classictone 40-18045. It has an 8 ohm tap rated at 15 watts.

Ok.. it will be a stage by stage analysis!

[Tom Phillips]

...As for the output transformer, I have read in other threads that the OTs on the older Gibson's are problematic. I'm wondering if I could use something like a Classictone 40-18045. It has an 8 ohm tap rated at 15 watts...

My 2¢ is that I have not found that to be true. Certainly not to the extent that I would proactively replace the OT if it appears to be working now. I recommend that you just leave it be unless there is a symptom that you have not mentioned that indicates a problem with the OT.

[Leo_Gnardo]

Good thing I had an extra Rectifier tube! The original couldn't produce the expected dc on the Cathode (350vdc).

You bet, that original rectifier wore itself out trying to charge up those duff old filter caps.

I'm with Tom on the OT, if it's working let it work, no need to replace.

[soundmasterg]

The original Gibson transformers were maybe a little undersized for the power of their amps, but I'm with the other guys...don't replace it unless it needs it. Your problems are likely resistors and caps. Check all the caps in the amp for leaking DC...you will likely find that all of the coupling caps are bad and leak enough DC to shift bias points on stages after them. Making sure the tubes are good is also a good idea so you know you are starting with a good base. For testing you can put a known good set of tubes in there if you have them. The EH12AY7 is a nice sounding tube and all there is for modern replacements. Vintage ones are better but are hard to find and expensive.

Greg

[TomCarlos]

Good-day Mates ...

I'm hoping I can have a check-in point and get a "Go No/Go" poll before I continue working on this amp. I'm wanting to break this down in stages and learn as I go along.

For now, I am working on the Mic/Instrument channel only (the top part of the circuit). I already know there is an issue with the other channel so I have removed the 5879 and 6SQ7 tubes.

Things I checked....

12AY7 preamp => Overall gain in that stage is around 14.

12AX7 Phase Inverter => Voltage across the 680 Cathode resistor is .79 volts, so that is apprx 1.16ma cathode current to each side.

6V6 Output Tubes => 279vdc on the screen grid (with no ac signal), 13.8vdc across the cathode resistor (20ma).

With the Tone control set to 12 O'Clock high, I am seeing close to 8vrm before clipping. As you can see in the photo, the bottom side of the waveform looks "tighter" when compared to the top. I realize this is not Stereo Hi-Fi, but I'm curious to know if you all have seen this type of waveform.

With an 8 ohm load, 8 Vrms, that is 8 watts rms. Does that sound right for this amp?

[guitician]

Ha! Looks like my avatar, but upside down. Compare your signal into the phase inverter to the output, and the signal from the inverter to see if the drive signal is normal first. 12-14 watts I think is "Normal" for these types of amps. And what is the plate voltage on the 6V6's? 13.8v vs. 15v on the schematic could mean weak tubes. What frequency is that? 1khz? Low frequencies will normally have less output. The transfer characteristic curve shows how that waveform occurs. I've seen it a lot from old tube amps, not "Clean" sounding ones, but what cranked old tube amp is "Clean".

[TomCarlos]

Thanks Jeff...

Good hint from you!

Yes, there appears to be an issue with the phase inverter. The incoming signal to the 12AX7 looks good. The output signals feeding the Output tubes "looks" good. The problem is that I am seeing only half the voltage swing coming out of the Phase Inverter that feeds into one of the Output tubes. So this probably explains the 8 watt output. I am going to check the circuitry around the 12AX7, including the tube socket, parts, and connections.

I hate this two sided PC board!!!!

[TomCarlos]

Update... getting closer...

I found a wiring error in the Phase Inverter circuit. So I fixed that. I have increased the output power but still have the problem of a mismatch of ac swing feeding the 6V6 tubes.

See attachment for measurements-

- 119 mVac coming from the Pre-Amp and feeding the P.I.
- 7 Vdc on the 680 ohm Cathode resistor = ~ 1.03 ma

On the top side of the 12AX7 (Pins 1, 2, 3)

94 Vdc on the Plate
8.33 Vac on the Plate
119 mVac on the Grid

On the bottom side of the 12AX7 (Pins 6, 7, 8)

100 Vdc on the Plate
10.7 Vac on the Plate
184 mVac on the Grid

I am wondering if the ratio of the 220K to 4.7K voltage divider (calculate during design) is off and that is the causing the imbalance.

I have swapped out several 12AX7 tubes thinking it might be that one side had more gain than the other- same results.

[guitician]

So did you set the AC input to 119mV because that is the point the output load wave would start clipping? How balanced are the two 220k resistors? Yeah, you could trim it to balance the PP drive signals, but how balanced are the 6V6's? I've always thought exactly balanced tube output sections were Hi-Fi, and not the classic old tube guitar amp.