Experts needed

[megadan]

tube amp experts, I know you're lurking somewhere.

 

I call upon the tone gods to summon you to me.

 

Originally posted by megadan I don't if we can arrive at an answer here, but maybe I can get some input from the tube gurus here. Inside my very Vintage Guild amp are, at the moment, 5 12AX7's, and 1 12AU7A. It's a two channel amp, and this is relevant, I think. There are nine tubes total. Of course, there are two power tubes because power tubes go in pairs. The odd tube out is the rectifier (?), because you only need 1 rectifier tube, obviously. That leaves 6 preamp tubes. It makes sense that there would be 3 tubes per channel. Now, what I'm getting at here, is that I'd like to replace the single 12Au7 tube with a 12AX7. The 12AU7 is a much lower gain tube compared to the 12AX7, which is basically the highest (see the chart: http://thetubestore.com/gainfactor.html ) Now normally numbers alone would not be enough to convince me to use the wrong type of tube, especially one of higher instead of lower gain. But here's the kicker: one of the channels is SIGNIFICANTLY quieter than the other. This channel needs to turned to 9 or 10 on the gain to get the proper volume. The other channel only needs 3 - 5 to get up to speed. This seems to be in line with the gain characteristics of the two types of tubes. Also, the layout of the amp puts the one 12AU7 right next to a 12AX7 in a pair of sockets. So, do you think I will be ok using a 12AX7 in there instead of the 12AU7A?

 

The amp in question is a 60's Guild Quantum/Thunderbird. Not much info out there about them.

Also, apparently that 7591A is NOT a rectifier.

What is this tube for???

[eSoTeRiK]

The 7591 is listed as a power tetrode. 3 power tubes?? It doesn't make sense.

I wouldn't necessarily assume that each channel uses 3 tubes. One of those (probably the 12AU7) is the phase splitter.

Is it possible for you to post some pics of it, inside and out? You're right that there isn't much info on this amp.

[Guttermouth]

7591a is usually used as a power tube.
putting a 12ax7 in that slot might cure your problem.it's likely that someone threw that au7 in to lower the gain in one of the channels.

[Cougar Hunter]

what?

post too large, can't read

[megadan]

(the two sockets you see are where the 12AX7 and the 12AU7A are)






From closet to farthest, left to right:

GE 6550A, 6550 (RCA 7317) (edit: the above picture shows them swapped, but they came and currently are in the opposite position, as I just described)

12AX7, 12AU7A

12AX7, 7591A


That's the "power amp section"

There are 3 more 12AX7's in the "preamp section"

(the two sections are connected by a cord you can see in the last picture)

[megadan]

bump for pics

[rushtallica]

Originally posted by megadan bump for pics

Make that a double bump.

[olejrod]

i'm not a tube amp expert by any means, but everything that i've found to read on this amp eluded me to believe that the power section is split into two, to seperately drive two preamps. is it possible that one power tube is bad or old, resulting in one channel being significantly quieter?

i'm not sure if that even helps answer your question, but from everything i read, that seemed to be what it was inferring.

like i said, i'm no expert at all. i hope someone can help you, they're apparently very rare and probably sound pretty cool.

[megadan]

Originally posted by Roccaforte Amps I would be very suspect about that 7591 even belonging in this amp. The octal socket its plugged into is probably for something else. Take an inside picture and post it here, I'll tell if it belongs there.

 

 

Do you want a better picture of the socket, then? It's tricky because only the front and back plates come off the amp, and it's a big amp. But I'll see what I can do.

 

 

Originally posted by Roccaforte Amps Now, as far as the rest of the tubes go; One is a driver tube, and the others are preamp, and does this amp have reverb or tremolo? The 6550 output tubes are not correct, it should be 6L6GC. So, the amp has been modified.

 

 

The amp has no reverb or trem or anything else. It's very very basic.

 

I believe the 6550's are correct - I know that the "other" version of this amp, the thunderbass, used smaller tubes. I have another more amp with 6 6L6 tubes, and that amp is not clearly louder than the Guild.

It's very very possible the amp has been modified, though, I don't really have any details on the original owner(s).

 

I see a lot of people claiming to have Quantums when they really have a Thunderbass... they used the same housing and face plates, so it makes sense. But all the Thunderbass pictures I've seen have shown a much smaller foot print with all the components much closer together.

 

here is the smaller Thunderbass:

http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&ih=003&item=130028322454&rd=1&sspagename=STRK%3AMEWA%3AIT&rd=1

 

 

Originally posted by Sri i'm not a tube amp expert by any means, but everything that i've found to read on this amp eluded me to believe that the power section is split into two, to seperately drive two preamps. is it possible that one power tube is bad or old, resulting in one channel being significantly quieter? i'm not sure if that even helps answer your question, but from everything i read, that seemed to be what it was inferring. like i said, i'm no expert at all. i hope someone can help you, they're apparently very rare and probably sound pretty cool.

 

 

Sri, your theory is very possible.

I measured the bias of each tube (which this amp is set up to do very easily, which is quite fortunate!!)

one tube measured around 25ma while the other was closer to 50!

 

 

But, this amp is so loud, it just seems louder than a single power tube could ever produce!

 

I can compare this to a vintage Fender Bassman, which I have played - the Bassman was much quieter... it basically reached the volume that my Guild puts out clean, under full clipping. That is, the Bassman with everything on 10, and completely distorted, was almost as loud as the Guild up about half and still clean or just overdriving.

 

I also compare it to a modern Peavey Alphabass (6 6L6's) and they are approximately equal in volume, though the Guild overdrives easier.

 

 

Here's a post from the Guild forum:

 

 

Hi megadan, The amp you have is more than a Guild Thunderbass. It is a Quantum! This amp uses a pair of 6550 finals and is the most powerful amp Guild ever made. It had the power of a 100 W Marshall plexi without the "presence"control. This looks like a Thunderbass only by the front name marker. Guild was always very thrifty on using common parts. However if you look inside this amp you will notice the amp is in two sections. You will also find a Quantum name plate on the power housing. The preamp control and power amp section are seperate. The size of the amp head is huge. If a conventional Thunderbass amp was mounted on top of your amp it would look quite small. The Quantum is 23" W x 9 1/2"H x 15" D. A Thunderbass is 21 1/2 W x 8"H x 11 3/8"D. Earlier Quantums were packaged in a Thunderbass cab and housing. These tended to overheat. The larger model was the last version. I just blew one of the 6550 finals running my Quantum through a Power Soak. Hope I didn't damage anything else. I just ordered a new set. matsickma

 

 

 

I'll take some more pictures tonight.

 

Thanks for all the help so far!

[megadan]

This was posted on another forum:

Re: Guild amp ------------------------------ I tried following the threads and looking at the pictures you published on the other forum. I also looked in my schematic sources and books for Guild amps. I couldn't find anything. I would guess that the 7591A power pentode was being used as a voltage regulator, possibly in tandem with the 12AU7. Trying to put out 100watts from two 6550 output tubes makes for some strange circuits. Although the plate voltage for a 6550 output tube can be as high as 600V or higher, the screen voltage must be relatively low, perhaps around 350V. The ordinary way to provide that reduced voltage to the screens is to use a series dropping resistor from the plate supply. But with the high power draw of a bass guitar amp, the current variation through that one power resistor can be extreme, and the consequent voltage variation at the screens of the 6550's will limit the power output. One way to provide constant 350 volts for the screens would be to use a power pentode like the 7591A as a series pass element in a voltage regulator. With 19 watts dissipation possible through the 7591A, the voltage at the screen would stay constant, regardless of the current draw of the output tubes. With regards to your specific problem of reduced output through one channel of your amp: It's all preamp stuff, which is to say, it's going to be in the first stage of the 12AX7 circuits. It could be a weak tube or plate or cathode resistors that have drifted out of specification. Guaranteed.

 

[olejrod]

i'm curious, what does the front look like? i've never seen the quantums front.

[megadan]

Sorry for the blurriness.
It apparently uses the same faceplate as the thunderbass, but it's quite a bit larger.

The most noticable difference I've seen is that every other Thunderbass I've seen so far has three switches on the front: power, polarity, standy.
Whereas this has only two: the power and polarity switch are combined.

[megadan]

Ahh, here's a better picture



You can see it's basically the same size as my 2x10!