Mesa Maverick problems please help

[Brymus]

Sorry for the long first post.
I have a Mesa Maverick 2 x 12 I got when someone needed gas money and never came back for it.
It started sounding farty,so first thing I replaced the tubes as it looked like the original tubes in it.
Actually sounded worse with all new JJ's
I discovered that the lead channel was bleeding through to the clean channel.
So I found the schematic and started trying to figure it out,I have built a few 15-20 watt PP amps myself.
Someone had replaced the 220uf cap on the PA with a 220R resistor :shock:
This had the effect of making the 75R cathode bias resistor 56R which explains why it was running so hot,IDK if it was on purpose or a case of bad eyesight,it could be mistaken for a axial cap, its an older resistor with no color codes just says 220R and the R is faint.

Anyway I am thinking the bleed through problem is from one or more of the vactrols.
Or even a bad relay/too.
One (vactrol) has been replaced,the first one next to the 2 x 220uf Mesa branded TVA caps.
The other 3 have 0795 printed on them.

THE MAIN QUESTION
Can someone tell me a cross reference for these vactrols?

The last person to work on this amp used about 100 zip ties (no kidding)

I really want to make it sing and keep it,as it's different from the usual "dual recto" Mesa's you usually see.A real hidden gem IMO.

So any help,suggestions on the above mentioned problem would be greatly appreciated.

It does sound OK by turning the lead volume all the way down,but that defeats the point of a channel switching amp.
I searched these forums without any luck.

 

[Monsta-Tone]

Hi,
Nice amp! Wish I could get one for gas money!
The LDR's (Vactrols) should be VTL-5C9
They can definitely cause bleed over.
There are 2 4N33 IC chips that I usually replace at the same time. These chips are opto-isolators that are used to switch multiple components (relays and LDR's) at the same time. Sometimes they go bad and cause odd things to happen.
I haven't seen any relays go bad in the sense that they would be half closed and cause bleed through. Usually they just get stuck or stop working altogether.

Let me know if you need a schematic or pictures of a functioning Maverick. andy at amps-tubes dot com

I would definitely set the bias circuit straight immediately!

 

[Brymus]

Thanks for the response Andy.
I really appreciate you taking the time to help.

Do you know the best place to get the vactrols and ICs?
Allied has the vactrols for about 7$ each IDK about the ICs though.

Also I measured the Mesa branded TVA caps,they are all in spec,but 20 years old.
(I only have a capacitance meter not an LCR to check the ESR)
I was thinking I might as well replace all the electrolytic caps while I have the PCB out since it is such a pain to remove.
Would that be what you would do?

Last question: would you change it to fix bias,my schematic shows a -50V lead on the PT but I am not sure if mine actually has it.
I could always make a bias tap off the HT.
Or I could make an adjustable cathode bias like mentioned in other threads on this board,using say a 125R 5W in series with a 100R 4W wirewound pot for each pair of EL84s?

I wouldn't be playing it loud enough to get the EL84s properly saturated anyway.

The part that eludes me is the best way of placing the 100R screen resistors in circuit with each tube.
To get an accurate bias measurement.
I am tempted to mount tube sockets to the chassis and ditch the PCB for the EL84 sockets.

 

[Monsta-Tone]

I usually just buy everything from Mesa. That way, my customers (don't really work on amps much anymore) get exactly the right parts. Especially when it comes to the filter caps. Only problem for me is that they only ship UPS Blue to Hawaii and it costs a lot! Two years ago, I ordered 4 mini switches for a customer's Roadking and the shipping was more than the parts! :mrgreen: Good news is, I'm glad I used the exact parts because I just traded for that amp a few weeks ago!

I would definitely change the caps while you have the amp open. I kind of look at it like changing the thermostat on my truck while I am changing the plugs. Just good preventative maintenance.

As for the bias, I don't know enough about cathode bias circuits to offer the correct advise. Sorry. I'd rather be honest than give bad advice that ends up in the death of a great amp or a good person.....
I usually adjust the resistor to the bias range I want and then just order tubes within that range. I do really like the idea of a separate resistor for each tube though, it would be awesome to be able to use any tubes you wanted. Not sure how it would sound, but sounds like a cool experiment.

Sorry I can't really be of more assistance.

 

[Brymus]

Thanks again for the response Andy,
For the bias,I was talking about a separate screen resistor(100R) for each tube.That way I could measure the dissipation of each tube.

Then they would all bias through a shared cathode resistor
(not adjustable w/o changing resistor),or each pair of of (push- pull)EL84s,
(2 pairs for the Maverick) would bias through a 5watt common cathode resistor in series with a 4 watt
wired wound pot.

That way each pair could be biased separately for better matching of current draw.
And also you could pull one pair to run at half power and it wouldn't alter the bias point.
Another advantage would be using two matched pairs vs a matched quad (dont know why you would want to though,except if you only had a matched pair of NOS)

Thanks again for the help,I will let you know how it ends up when I finish.

I could also post some pics if anyone is interested,but there are pics on the NET of Maverick guts.