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I just bought a ga-5 (early sixties) (6v6,12ax7,5y3) amp off of e-bay. I am in the process of checking it out. The filter caps were already changed. I notice a raspy overtone when I strike a note. I hooked up a spare 10" speaker to see if there was an improvement (seemed to be better). I have ordered a new 8" speaker from Weber. I also have ordered caps and resistors to freshen the old gal up. My question is this. I was playing the amp at low volume last night. I noticed that the control panel was hot to the touch. I usually play the amp wide open and it only gets warm. What would cause a amp to heat up at low volume? Thanks
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How hot is hot? Small tube amps such at this don't have much surface area on the chassis to dissipate heat and the panel should get warm. If it's too hot to touch then you've probably got an issue that's heating up the power tranny - which will sink heat directly to the chassis - or perhaps the OT. Play the amp for a while and then touch the PT (unplug the amp - don't touch anything under the chassis) - if it's "slightly uncomfortable" it's probably OK. If you can't keep your finger on if for over a minute you should take it in for service. This is a Class A amp so the heat dissipation is pretty constant all of the time even if there is no signal and it will get warm. 'Tis possible that the 6V6 is drawing too much current but you'd probably notice blatty sound and red plating before the current draw would heat the OT -> heat the chassis.
Rob -
Rob,
The amp had replaced filter caps when I bought it. It was operating OK but there would be a click/snap once in a while. I changed the other caps and some of the resistors. The temp is OK now (the tubes never turned red). I have a weber 8" alnico on order. The amp runs strong now with reasonable heat. I do have an issue with ghost notes or fiz right now (even at low volume. I'll see if the new speaker changes this. I have also tried a new set of tubes. The amp has a nice overdriven tone.
TomComment
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Working great
The new Weber speaker along with the electrical parts renewal really seems to have done the job. The amp is strong,quiet,and has a nice sound. I was wondering if anyone knows how to get a little more treble sound out of this circuit. I may try a treble boost pedal to see what happens.Comment
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These amps do traditionally sound pretty mellow. Depending on what "a little more" treble means to you, the following ideas may do the trick:
1) Try adding a small bright cap across the volume pot upper lug to the center lug. This may do the trick if all you need is a little more sparkle. I'd start with 47pF and adjust up or down to taste with the max being around 120pF. The magnitude of the effect will depend on how high you usually turn up the volume control. When you turn up to "10" the bright cap is effectively out of the circuit.
2) You could try reducing the value of the 20uF cathode bypass cap on the first stage pre-amp triode. I'd suggest 10uF then 5uF then maybe 1uF. The tradeoff is less bass fullness and a drop off in overall gain.
3) You can also use a combination of 1 & 2. The mods will be interactive so you can iterate to get the combination that sounds best to you.
I'd be interested to know how it works out for you.
Regards,
TomComment
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Tried the bright cap
Tom,
I tried your idea. I had a 68pf cap around so I put it in. Have a look at the pictures to see if I did it per your instructions. Hard to tell on the bench if I have more treble (seems so). I think I will keep the cap in for a while, then take it out to compare.
(http://home.rochester.rr.com/thollar/ga5_1.jpg)
(http://home.rochester.rr.com/thollar/ga5_2.jpg)
Thanks,
TomComment
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Looks correct to me. The Bright cap goes between the vol pot center lug and the outside lug that is NOT grounded. Your volume pot has a white wire connected from the grounded lug to somewhere near the tube socket. I'm assuming that it goes to a ground point.Comment
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