Changing 7591A tubes in my maverick

Dreamlander

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Well... I got my electro harmonix 7591A tubes today so i decided to put them in, They are much larger than the old ones, but i didn't think too much of it. So i put them in and fired it up.... nothing but a little hummm. So i am not sure if they just need to be biased or if i really destroyed something. :oops: John?
I am thinking i might have to make a trip to the amp tech. :roll:

7591Atubes002.jpg


7591Atubes001.jpg
 

matsickma

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Hi Dreamlander,

The EH 7591A look big compared to the standard 7591A because to save manufacturing costs and tooling they use a 6L6 bottle. Pin outs are correct for a 7591A. I bought a matched quad set a few years back and I tried them in a Maverick, Thunderbird and Thunderstar amp. The worked fine in my applications. (Note: The Thunderstar later had a Cap problem but it was not related to any tube issue.)

M
 

capnjuan

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Hi DL....dang.... :evil: let me look at some stuff; be back shortly ... dang .. :evil: cj
 

capnjuan

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put the old tubes back in, fire up, does it work? Yes, i know was crappy before but as crappy as it was?
 

capnjuan

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Hi DL; does the amp work with the old tubes?

See pic below: near the output tubes, do you have the little pots indicated in the red circles showing on the tube deck? On the right; do you have the little fitting on the back of the chassis; it's a test point; the left hole is for the left tube, and right for right; do you have a test point on your amp?

bassbias02.jpg
 

capnjuan

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Hi DL; not hearing anything back; you're either red-faced annoyed or you've got it running. In either event, I'll be around tomorrow. The pics are to determine whether you have bias pots and a test point. If yes, you can adjust bias and maybe fix things if bias is the issue. If you don't have the pots/test point, then you can't adjust bias although, if you have fixed bias, is less likely to be the source of your issues anyway.

Hope the new tubes are working.

John
 

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The JJ's are the correct size, but the EH are electrically identical.
Hum is curious.... :|
 

Dreamlander

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Well... they seem to work fine after i tried them a second time. :p I think it may have been a guitar issue the first time i tried. :oops: I just didn't have much patience and was worried about hurting the amp.
This amp has a hum balance pot on it. Which must be part of biasing the tubes. Not to sure. This is what Danerectal informed me of anyway. We turned the knob to the point of having the least hum? Well it works anyway and sounds MUCH, MUCH, better. Way more attack at high volum and breaks up beautifully.Those old tubes must have been fried. I can't way to get that weber speaker in there. :mrgreen:

Thanks everyone.
 

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:D :D :D :D :D :D :D

The hum balance has nothing to do with biasing the tubes. It balances the heater current in the tubes. If it's cranked to one side, it's going to make the amp hum like a sunafagun. It's cathode biased, so you don't have to worry about that.

Speaking of clips, I finally have my webserver up and running. I want to make sure it's secure and then I get to host my own pics and stuff!

BOT, glad to hear it, Dreamlander!
 

capnjuan

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Dreamlander said:
Well... they seem to work fine .... This amp has a hum balance pot on it. Which must be part of biasing the tubes Hi DL: Default is correct, it just balances the energy to operate the heaters or filaments. This is what Danerectal informed me of anyway. Don't lose phone the Great Dane's phone #. We turned the knob to the point of having the least hum? Award yourself a cold beer. Well it works anyway and sounds MUCH, MUCH, better. 8) 8) 8) Way more attack at high volum and breaks up beautifully.Those old tubes must have been fried. I can't way to get that weber speaker in there. :mrgreen: Thanks everyone.
You need to tell me whether the pics match your amp; we aren't web-cam enabled yet; I can't see what you see; please confirm. Can the Great Dane help you with biasing? It isn't any big deal but if you'll tell me about the pics, the rest of it is pretty simple.

Congratulations on your amp! John
 

Dreamlander

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Here is a pic of mine. I don't see any little pots or the test point on my amp.

7591Atubes.jpg


This will be my last post for a while. Gone for a week.
 

capnjuan

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Thanks DL; okay, your amp is 'fixed bias'; output tubes cathodes tied together and to ground via parallel resistor/capacitor. Good news; no screwing with bias; bad news; no screwing with bias. As the tubes age, they will wear out at slightly different rates. As a result and over time, one can lag behind the other causing some sonic trash at a minimum; this is only after extensive use.

If you have the pots, you can tweak bias so the tubes operate more or less uniformly. Also and for any amp lacking the pots/test point, it means that re-tubing requires matched tubes; with bias adjustment, you can use unmatched tubes; a convenience if you lose one.

Glad it's working ok ... betcha you were white as a ghost when that amp didn't power up ..... :lol: :lol: :lol: :lol: :lol: not teasing really ... just enjoying the moment ... I've been there in one form or another more times than I care to remember! cj
 

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I really like the tone out of a cranked amp with 7591A finals-my favorite. The Maverick guitar amp breaks up very nicely. The Maverick Bass amp doesn't appear to have as much gain as the guitar amp. One of the issues I have with the Maverick amp is that the tone is really outstanding but I usually like a bit more volume. Pushing the amp to the MAX position tends to go overboard and the amp doesn't have a power supply to handle the surge of sharp guitar attack notes. To overcome this issue I reduce the volume down to around 3'oclock and then mic the Maverick through a Guild Thunderstar Bass amp and cab with 2-15" JBL E130's! I route the mic through a Volume pedal to offer immediate contrpol of the overall volume. This combination is playing Heaven down in my basement when no one is home! :D

I often felt that that the perfect amp for me would be a Guild like amp with 4 -7591A finals. I have seen an Audio Guild amp with 4 -7591A. However a few years back I tried a Audio Guild amp with a pair of 7591A and wasn't impressed with the tone. It was a lower gain amp and lacked brightness- i suspect orientated toward Jazz.

Does anyone know of other amps that use a quad of 7591A? Ampeg used to be a user of 7591A's. any quad models?

M
 

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None, to my knowledge. Maybe an oddball hifi model, but I've never seen or heard of a 4 7591 musical instrument amp.

edit: Maybe an Ampeg echo-twin?
 

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Lucky is not such a bad thing. :) You can gain skills, but constant bad luck stinks.
matsickma, you want to look for an Ampeg Super Echotwin. That's the only 4 7591 amp I know of.
 
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