preamp tubes in a Quantum

Hi all

you're all very helpful folks when it comes to guild amps... and I don't really know where else to turn!

Here's a little problem I'm having with my Quantum:

Inside the amp at the moment are 9 tubes. 2 power tubes (6650/6550A), one 7591A (???), and 6 preamp tubes: 5 12AX7's, and 1 12AU7A.

It's a two channel amp, of course, and it would seem to make sense that there would be 3 tubes per channel.

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: the first channel 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?


* it looks something like this: 3 12AX7's spaced evenly through the "preamp section" of the amp, ie, the faceplate where the knobs etc are.
moving into the power section, there's a 12AX7 right next to the 7591A.
Then next up is the 12AX7 and 12AU7A pair.
Then finally the power tubes.

Hi megadan,

Your later series Quantum amp only uses three 12AX7 tubes in the dual channel/Master Vol section of the amp. The small signal tubes (12AU7 & 12AX7) that are attached to the power amp housing are used to boost the signal AFTER the Master Vol control. Changing the 12AU7 to a higher gain tube will not fix the problem you are having with the Bass channel being significantly lower in gain than the Normal channel.

I have had a similar problem with a Thunderbird amp and the low volume problem was due to a faulty three tone switch (i.e, the little slide switch that select different tone characteristics of the amp). You might try jogging the three position swithch and see if the sound level jumps around. I have used a little bit of Radio Shack tuner contact cleaner on these switches but the best things is to replace the switch.

With regard to swapping the 12AU7 with a higher gain tube like a 12AX7...the link you provided gave a good idea on substitution. I think it would be best to work your way up in stages. Try the next level tube to see how that works.

I am very courious how a higher gain tube will alter the tone of the amp.

Also, your Quantum amp is the later series design. I am trying to collect serial number info on these amps. Can you send me a Private Message on your amps serial number?

Thanks,

M

Thanks matsickma

I just ordered a whole new set of tubes. I'm going to replace them as they are for now and see what happens. I'll also check the switch on the bass channel.


Oh - there's been a lot of debate about what exactly the single 7591 power tube in there is for - do you have any idea?


I will send you a PM later this evening when I get home!

Dan

Hi megadan,

My guess is that it is a driver tube for the 6550 finals. By this I mean the single 7591 may be an intermediate stage between the lower signal level preamp stage tubes (12AU7 & 12AX7) and the 6550 finals. The single 7591 maybe operated in Class A mode and the output providing the boosted signal to the 6550's.

This is just a guess, I am not well versed on circuit design but may know enough to verify that the output of the 7591 is driving the 6550's. One of these days I'll pull the power amp section out of the Head and see if I can make sense of the configuration.

I'll let you know.

M

Thanks!


I would love to know what the original compliment of tubes in this head was. When I replaced the preamp tubes, I found what I originally thought was a 12AX7 was actually something called a 7247, aka 12DW7 (which is actually 1/2 a 12AU7 and 1/2 a 12AX7). I replaced it with a 12AX7 and there doesn't seem to be much change, except the bass channel might be a toucher cleaner now, but that's probably just the other preamp tubes being replaced as well.
It's very likely that somewhere along the way, someone just replaced bad tubes in this with whatever they had lying around. So it would be cool to see an original schematic, though no one seems to have one!

oh! Check your PM's, I sent you one.

My amp is like new and looks origional. It also has a 7247.

The tube lay out of my Quantum is as follows:
Pre amp housing: 3-12AX7's.
Power amp housing (standing behind the power amp housing and starting from the front of the amp):
"Front row": on-left: 1-7247, on-right: 7591
"Middle row": on-left: 12AU7, on-right: 12AX7
"Back row": on-left: 6550, on-right: 6550

I haven't been able to track down a schematic.

M

matsickma said:

My amp is like new and looks origional. It also has a 7247.

The tube lay out of my Quantum is as follows:
Pre amp housing: 3-12AX7's.
Power amp housing (standing behind the power amp housing and starting from the front of the amp):
"Front row": on-left: 1-7247, on-right: 7591
"Middle row": on-left: 12AU7, on-right: 12AX7
"Back row": on-left: 6550, on-right: 6550

I haven't been able to track down a schematic.

M

Click to expand...

That's almost exactly like my amp, except in the middle row, I had a 12AX7 on the left and the 12AU7A on the right....

Hmm. I wonder if the order of these tubes is significant?

Hi Megadan,

I re-checked the tube arrangement on the power amp section of my 6550 Quantum. The 12AX7 is behind the 7591 tube and the 12AU7 is behind the 7247. This description is less ambigious than my earlier post.

M