Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: unclerny on October 11, 2012, 01:53:46 pm
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Hi Forum. I've been working on this Ensign now longer than I should. You won't find an accurate schematic because it's two amps put together from the factory. You have to reference the Gibson Ga-55rvt and the Ga-45rvt and use the circuit board from the 55 and the chassis stuff from the 45. Don't ask but it's what is in there.
I've been troubling over the low output it has. I've checked all the voltages and they are excellent. New tubes for 6l6 and 6eu7 and known good ones in the oa2 and 6fq7. It's simply not loud.
I noticed that the two 12s were wired in parallel for a 4 ohm load. The 55rvt doesn't have this setup and calls for an 8 ohm load. But the 45 does but the speakers are in series, 16 ohm, and a 16 ohm OT. Neither of the drawings are very good and it's hard to tell OT model number.
OK, the question.
If this was meant for a 16 ohm load and was wired all these years for a 4 ohm load could it eventually hurt the OT to produce low volume?
Any feedback would be great.
Thanks Tim
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OK, the question.
If this was meant for a 16 ohm load and was wired all these years for a 4 ohm load could it eventually hurt the OT to produce low volume?
Probably not. At least, not without other serious abuse.
Have you simply tried a different speaker/cabinet with it?
Does your amp have ~-50v on the grids of the 6L6's? Have you applied a suitable test signal to the input jacks and measured the resulting a.c. voltage at the output tube grids? Is it close to 50v peak at some reasonably high (7-Max) volume setting?
If the bias is some other value, does it make sense in light of the GA-55 6L6 bias and plate/screen voltage? Does the volume test result in a peak a.c. voltage of approximately the bias at max/near-max volumes?
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A higher speaker impedance can damage tubes and output transformer but you probably hear a poor sound , not only low volume.
You should check Phase Inverter circuit . Problem could be there or a signal capacitor defective
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Thank you guys. I'm glad to hear that it's unlikely the OT, not exactly the easiest part to replace and finding a proper replacement/ cost. I actually had it running off my known good speaker cab and the amps cab and I'm not impressed that either are nearly as loud as it should be.
I know for a fact that the PA bias is there and I did adjust the bias. It's 460 Vb with 45m.a. I measured that negative voltage prior to adjustment and it actually was -50.1 volt. This was drawing about 32m.a.
I'll do those a.c. voltage peaks when I get back to the shop on Tuesday. Two jobs and all.
I'll also do the fine tooth comb thing on the entire preamp. I've probably missed something. These things are a tangle inside.
Regards Tim
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Agree with Stratele that if the bias seems correct, and you get something approaching proper drive voltage, then you probably have a leaking coupling cap somewhere.
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Agree with Stratele that if the bias seems correct, and you get something approaching proper drive voltage, then you probably have a leaking coupling cap somewhere.
Don't fotget to check Phase Inverter circuit too.
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Reputedly that's the amp that Roger McGuinn :worthy1: recorded "Mr Tambourine Man" with the Byrds :icon_biggrin:
Perhaps the Gibson / Epiphone answer to a Vibroux Reverb! :w2: