Gibson GA8T build.

[Coastie99]

Hello from a new member in New Zealand.  I've recently started scratch-building amps.  Learning fast, but still ( very ) technically challenged !

My problem is, finding a suitable PT and OT for my current project, a Gibson GA8T clone ....... http://www.schematicheaven.com/gibsonamps/ga8t.pdf

This version of the GA8 uses 6BM8 / ECL82 valves .......... http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_1/6BM8_ECL82.PDF

I've managed to transcribe the schematic into a ( hopefully correct ) layout drawing and have started my project.
My problem next, is my lack of technical knowledge.  To whit, I cannot find the correct transformers.  I know all the places to look and have done so, but with no luck. Or at least, anything I can recognize with my limited technical knowledge.

There is a GA8T OT at MM, with 8K primary.  I don't know if this is suitable although I assume that it would be, given MM's good reputation.

I don't want to end up spending moon-money on transformers, but would happily install a MM OT and another manufacturer's ( less expensive ) PT.

So, I have two questions:  Is the MM OT right ?  And, can I use an "over-rated" PT and "juggle" the dropping resistors to come up with the value that I require ?  Better still, of course, is there a standard PT that will do the job !

Any help would be hugely appreciated.

Thank you,

Gary.

[tubenit]

can I use an "over-rated" PT and "juggle" the dropping resistors to come up with the value that I require

Yes, that will work.  Look for a PT that is around 250-0-250  .......... to  275-0-275 range with enough ma current handling and then work with the dropping resistors. A PT and OT that one might use for a 18w might be a consideration? I used 8 ohm speakers with 6BM8 in a push/pull.

Using a vintage (not Sovtek) 5Y3, you should end up with VAC X 1.1 (sort of).   So with a 250-0-250, you should end up with around 275v on the plates.

My Little Wing Reverb and my 56T which used 6BM8 tubes had about 305v on the 6BM8 pentode plates. I think DaGeezer may have had even higher voltage on his Little Wing.  

With respect, Tubenit

[billcreller]

 I seem to remember that there were two GA8T schematics, but my memory doesn't work right now. Output tubes were different, like EL84s one one version, and something else on the other. (?)

  The one I had used the EL84s..   I replaced the coupling caps for a larger value to get it a bit more mellow/less brassy.

[VMS]

Hi,

if I'm reading that voltage chart correctly (280VAC), then my guestimation for a suitable PT would be Hammond 371X.

63VA
560V C.T. @ 69ma.
5V C.T. @ 2A
6.3V C.T. @ 2A

Though I'm not 100 percent sure if that 69ma is enough.

[tubenit]

I used a PT that had 150ma.   Looking up my old voltages on the Little Wing Reverb.  The push/pull pair of 6BM8's were drawing about 30ma each.

To be on the safe side, I'd want a PT that has closer to 90- 100 ma if possible. You might be OK with 70ma with this build?
The advantage of a PT that has more ma current handling is that IF you ever needed to convert the amp to have more wattage to use EL84's then you'd have the PT capability already.

With respect, Tubenit

[Coastie99]

can I use an "over-rated" PT and "juggle" the dropping resistors to come up with the value that I require

Yes, that will work.  Look for a PT that is around 250-0-250  .......... to  275-0-275 range with enough ma current handling and then work with the dropping resistors. A PT and OT that one might use for a 18w might be a consideration? I used 8 ohm speakers with 6BM8 in a push/pull.

Using a vintage (not Sovtek) 5Y3, you should end up with VAC X 1.1 (sort of).   So with a 250-0-250, you should end up with around 275v on the plates.

My Little Wing Reverb and my 56T which used 6BM8 tubes had about 305v on the 6BM8 pentode plates. I think DaGeezer may have had even higher voltage on his Little Wing.  

With respect, Tubenit

Tubenit, thank you for that information.

Would a Weber W025130 be satisfactory ?  https://taweber.powweb.com/store/025130intsch.jpg

{edit- misplaced /quote --PRR}

[tubenit]

Yeah, that should work OK using the 270-0-270 windings.  


 I was going to suggest maybe Hoffman's 18w PT ............ however........

Uh, I  didn't think thru that real well ................ you probably don't have 110v right?  And need international voltages as an option?

With respect, Tubenit

[Coastie99]

Yeah, that should work OK using the 270-0-270 windings.  


 I was going to suggest maybe Hoffman's 18w PT ............ however........

Uh, I  didn't think thru that real well ................ you probably don't have 110v right?  And need international voltages as an option?

With respect, Tubenit

Yessir: 230V.

And, the next question:  Is there a "formula" for deducing dropping resistor values, or is this a suck-it-and-see proposition ?

Not too technical please 

Gary.

[Coastie99]

OK, I have the PT sorted out, and I thank you all for your replies and help.

Now, what I've always thought would be my bigger problem ........ the OT.

Suggestions please ??

Thank you,

Gary.

[tubenit]

I've probably built close to 20 different builds with most being non-clone builds.

I am sure there are formulas for the B+ rails.  However, I don't have a clue as to what they are.  

I just look at other schematics that have some similarities,  make my own guesses based on experience, then
tweak things until it sounds like I want it to.  Experience & experimenting has brought me to certain preferences on plate voltages  for example.

If I were in your position, I'd probably simply increase the one resistor between node "B" (6BM8 screen) & node "C" (phase invertor) and leave the rest alone and try that.  (NOTE:  node B and node C is simply my way of describing an order).

I'd try that and then adjust to what you like by changing B+ dropping resistors or plate/cathode values.

I'd probably use an 8 ohm OT that would work for a Princeton Reverb, Deluxe Reverb, 18 watt, 5E3 Deluxe, etc........ something in that range.

With respect, Tubenit

[bluesbear]

The EL-84 version of the GA-8t had 299V to the plate, according to the schematic provided in the official Gibson operations manual. I actually have one, believe it or not. I've always thought that 300V is just about perfect for EL-84's.
Dave

[Steve_P]

Hey Gary!
According to the schemo for the u45B, the Japanese used 16K to 8 ohm OT's.
Seems a little extreme, but I am still on holiday, so I haven't had a chance to fire it up but the onest.

[Coastie99]

Well, it seems like I should do this ............

Buy the MM OT and a suitable PT, get this thing going, and start measuring !

A very big thank you to everyone who provided assistance.

Gary.

[FYL]

I was going to suggest maybe Hoffman's 18w PT ............ however........
Uh, I  didn't think thru that real well ................ you probably don't have 110v right?  And need international voltages as an option?

The Heyboer 18w PT sold by Doug is the 120/240V version.

[Coastie99]

Yes, I spotted that.

Unfortunately, my build ( already started ) doesn't allow for a lay-down transformer.

Gary.

[tubeswell]

Hi Gary - check your PMs - Pete