Need help with Gibson GA-5T

[Chuck H]

"Now I am getting the full 3.5 watts voltage from the wiper of R10 to the grid of the 6C4."

Good, perfect.

"it definitely has more volume. Not Blues Jr. type of volume but much more than it had originally."

Have you tried both amps through the same speaker? Speaker eficiency can play a big role in overall volume. That amp should be about 3/4 as loud as the Junior through the same speaker. Otherwise there is still work to do.

"I do notice a bit of crappy distortion on the transients. It sounds like tube noise to me." AND "The Sovtek 6eu7 replacement I bought has some issues so I am limited to using the NOS 6EU7 that came with the amp."

Can you be more specific?


"Do you think that there is something else up with the voltages I have posted?" AND "Now I am getting 45volts on the plate of the 6C4 and 45/53volts on the grid of the EL84's."

Now that you've repaired the power supply and there is some signal getting through the amp it's probably time test the DC voltages. But 45 and 53 volts AC on the grids of the EL84s would be too much and seems odd to me. We may get back to that.

"I can't get a reading on the plates of the EL84's."

Thats because there shouldn't be any AC there. The voltages indicated on the schem are DC volts. They are marked for location. Test and compare.

Chuck

[vortex]

Now I am getting the full 3.5 watts voltage from the wiper of R10 to the grid of the 6C4."

Good, perfect.

Yes , many thanks to both you and Enzo!

"it definitely has more volume. Not Blues Jr. type of volume but much more than it had originally."

Have you tried both amps through the same speaker? Speaker eficiency can play a big role in overall volume. That amp should be about 3/4 as loud as the Junior through the same speaker. Otherwise there is still work to do.

This is a subjective opinion on my part. I've cranked a friends Blues Jr. in a small studio room and it was "I should get some earplugs" loud. I used the Blues Jr. as a comparison due to the similar output tube configuration. I am realistic and realize that there are going to be differences in the circuit design and components between the two. If this Gibson is rated at 12.5 watts I would say that it is still lacking in poop. Right now I would compare it to a Vibro champ in volume. I have to say that the amp sounds good, nice chime and depth. It doesn't get any kind of wound up overdrive on 10, perhaps this was Gibsons intention...
I did test the amp with it's original 10" speaker, which is a good sounding alnico but I switched up to a 1x12 cab with a 8 ohm Celestion 80 (fairly efficient) that I am more familiar with for a better impression of the amps range.

"I do notice a bit of crappy distortion on the transients. It sounds like tube noise to me." AND "The Sovtek 6eu7 replacement I bought has some issues so I am limited to using the NOS 6EU7 that came with the amp."

Can you be more specific?

I tested the amp with a different guitar today and wasn't able to duplicate the transient distortion. What I was hearing last night sounded like some gain stage was being hit too hard and there was a bit of "kacking" on the transients. Bad distortion, not good distortion...I've had this happen with other amps before and it was fixed by swapping out the preamp tubes. Good results today but I will keep on top of this. The test guitars were both decent strats with single coil pu's.

Regarding the "new" Sovtek 6EU7, when I swapped it in, it was immediately noisy. Also a bit disturbing was that the Sovtek's pins seemed to be slightly wider spaced and was not a smooth fit for the socket. I've seen this before with an old Ampeg Reverborocket where the Sovtek 7591 tubes fit poorly and were prone to falling out! A NOS 7591 fit like a glove...

"Do you think that there is something else up with the voltages I have posted?" AND "Now I am getting 45volts on the plate of the 6C4 and 45/53volts on the grid of the EL84's."

Now that you've repaired the power supply and there is some signal getting through the amp it's probably time test the DC voltages. But 45 and 53 volts AC on the grids of the EL84s would be too much and seems odd to me. We may get back to that.

I've gone through and documented the DC voltages , see below. DC indicated in Red, AC in blue. The 45 and 53 volt readings on the EL84 grids are with the 0.5 volt signal input and the Vol maxed. In general the DC voltage readings look promising with the exception of the tremolo section. Please have a look and let me know what you think and if there is any other information I can provide.

[Chuck H]

"If this Gibson is rated at 12.5 watts I would say that it is still lacking in poop. Right now I would compare it to a Vibro champ in volume."

Many Gibsons are not very loud. Not sure why. But I built an amp not long ago using salvaged parts. The PT puts about 300 volts on the plates of a pair of 6V6s. The OT is quite small. About the size of three nine volt batteries placed beside each other. I'm pretty sure that it's straight wound too (= no interleaves). My point is this. The amp I built is similar enough. Probably around 12 watts, kinda cheesy OT and lowish plate voltage. That amp is about twice as loud as a VibroChamp. By the schem I would think yours would be too. I just worked on a VC for a friend and as usual it was about as loud as a shout.

One thing still looks funny (to me anyway). The high AC on the power tube grids. At full clip I would expect to see less than 25 volts on each of of those grids. From a design perspective you can really pound on power tube grids when using transformer as a phase inverter. But in that era most designers didn't want power tube distortion. My guess is that in your circuit those power tubes should start to distort with about 10 volts or less on those grids. So, considering the amp, 50 volts seems insanely high. But I've never designed with an interstage transformer before or ran a trem into one for bias modulation. So I could be missing something here. Perhaps Enzo will see this and help out.

By the way, How well does the trem work? Could be key. Also, What is the DC on the ungrounded end of R19 with the amp idling and the trem off?

Chuck

[vortex]

By the way, How well does the trem work? Could be key.

Chuck

The trem works well and sounds quite beautiful. A nice warm pulse.

Also, What is the DC on the ungrounded end of R19 with the amp idling and the trem off?

I measure 10.5 volts.

[Chuck H]

Well...Hmmm.

Everything looks fine with the DC voltage. The bias point is near dead on, but we can't let that fool us. so:

Are there any circuits built into the amp that arent on the schem. I'm looking for a negative feedback loop that would be connected to the speaker output via a resistor.

And:

Check C12 to make sure it's in place and in good working order. You would need a meter that can read over 20uf capacitance to do this. Otherwise you could just replace C12 with a new cap to be sure. I might use a 47 or 100uf for that instead of 20uf. If that cap is missing or open it would reduce gain. It's a polarized cap so you need to make sure the + is on the tube socket end and the - is to ground.

And:

How old are the EL84s? Pull one of the power tubes and remeasure the DC on the ungrounded end of R19 with the amp idling and so signal input. Then swap the power tubes so the other one is out and measure again. If it turns out that one tube is doing most of the work you could get much louder with good matched tubes.

Thats all I have for now.

Chuck

[vortex]

Are there any circuits built into the amp that arent on the schem. I'm looking for a negative feedback loop that would be connected to the speaker output via a resistor.

No extra circuits, the amp matches the schem.

Check C12 to make sure it's in place and in good working order. You would need a meter that can read over 20uf capacitance to do this. Otherwise you could just replace C12 with a new cap to be sure. I might use a 47 or 100uf for that instead of 20uf. If that cap is missing or open it would reduce gain. It's a polarized cap so you need to make sure the + is on the tube socket end and the - is to ground.

I did replace the C12 when I redid the power supply. Orientation is correct. If I replaced the 20uF with a higher value what would be the result?

How old are the EL84s? Pull one of the power tubes and remeasure the DC on the ungrounded end of R19 with the amp idling and so signal input. Then swap the power tubes so the other one is out and measure again. If it turns out that one tube is doing most of the work you could get much louder with good matched tubes.

I did this test and measured 8.5 volts with one tube and 9 volts with the other. I did swap in a new set of matched EL84s and there wasn't any appreciable volume increase.

Maybe this is all the volume I am going to get out of this amp. The amp sounds good.

Once again, thanks for all your help, much appreciated!