New user, MKII A question/ troubleshooting

[obergfey]

Hello,

I'm new to this boeard and alos to mesa/boogie amps, but this morning I randomly tried a used MKII and I took it home for 600€. Non reverb, no eq. The amp's serial number is 4546 and I have some quesions about it:

There are a couple of oddities: first, the hole that says "reverb" and that is pusually plugged on non reverb amps is actually a jack hole. Then on the underside of the chassis there is a rca input. I don't know what they are for. I don't understand how to use the effects loop since I only have two outputs: power amp in slave.

Second concern: I find that the clean channel is very quiet. I would assume that 60W at full blast would be unbearable, but it is, and I'm even afraid that It wouldn't stand up against a drummer. I don't know if it is related, but the output tubes (6L6) run very cold in my opinion, I am actually able to touch them without burning my fingers.

Thank you for your help!!

 

[Boogiebabies]

I have no idea what the input jack in the reverb slot is for without pictures.
If it has 5 preamp tubes and RCA jacks on the bottom side of the chassis it may have reverb, but your missing the tank. The power tubes are a definite issue. They should be blazing hot after a minute or so. The tubes could be dead, super cold plus fan cooled or your bias circuit has been screwed up. A few pictures of the circuit boards may allow us to see if it's been altered with a bias trim pot and to see what that janky footswitch is for.

 

[obergfey]

The "reverb" jack is hooked up to the lead master and master 1 knobs. The rca input looks like its hooked up to the middle knob, among other things.


????????

I'm posting pictures really soon. within 10 min.

 

[obergfey]

Here are the pictures

http://indiana.facebook.com/album.php?aid=2281342&l=61d04&id=6857287

 

[Boogiebabies]

Yann,

You might want to cut back on the coffee. I can't see too much with those pictures.

 

[obergfey]

Could you tell me exactly what I need to take a picture of?

 

[kaluablue]

Hi all

I'm new here too, and I just thought I could help a little here about the Mk11a.

The RCA Jack on the bottom is a footswitch connetctor for the Gain Boost (Master1 Pull) Feature
On the non-reverb MK11a the Gain Boost switch is active only in the lead mode.

The effects loop return is the 'PreampOut/PWRAmp In' jack on the rear, the send is on the bottom of the chassis, behind the powertubes. This loop is placed behind the Mastervolume and thus is quite useless, due to the very high signal levels at this point.

The 'reverb' jack on the rear is obviously not original, there normally is plastic cap in this hole.

I'm a MK11a (non EQ, non reverb) owner myself, and have gone through the schematic (Aspen Pitmans Tube Amp Book) again and agin ...

greetz, Dieter

 

[kaluablue]

Hi again
Something can already be seen on those photos (the first one,actually) :
the 'reverb'jack is the 'send' jack moved from the bottom to the rear.

Greetz, Dieter

 

[obergfey]

I'm wondering if the reverb jack is not the effect loop jack that's been moved to a more convenient location.

 

[Boogiebabies]

The wire coming off the pot to the jack on the back looks like it is going to the phase inverter. I don't exactly get what is going on with the RCA jack. The factory schematics will not necessarily help. Sometimes you have to sit down and diagram what is going to the plates, grid and cathode. In this case, those Fetrons could be shot as well. Some clear pictures would help.

 

[Restless Rocks]

The "reverb" jack is hooked up to the lead master and master 1 knobs. The rca input looks like its hooked up to the middle knob, among other things.



This sounds like a jack installed to add a non FX loop volume pedal. I have installed a mod like this on my own amps so that my volume pedal acts exactly like the master volume without affecting the fx return level and only requires one cable to the pedal. Could also be used as a preamp send to drive another power amp.

 

[Boogiebabies]

The gain boost is a .005 cap switched in to the mid pot. This may be the RCA jack on bottom of the chassis.

 

[kaluablue]

the .005 cap is on the ground side of the Mid pot.
The gain boost switch opens the ground connection of the mid pot. This actually disables the tone stage completely and results in the boost. The cap just darkens the sound then, I removed it.
The Footswitch (RCA jack) is only active with 'Gain Boost' pulled out. It then shortens the Mid Pot to ground again, which switches the gain boost out.

 

[Boogiebabies]

the .005 cap is on the ground side of the Mid pot.
The gain boost switch opens the ground connection of the mid pot. This actually disables the tone stage completely and results in the boost. The cap just darkens the sound then, I removed it.
The Footswitch (RCA jack) is only active with 'Gain Boost' pulled out. It then shortens the Mid Pot to ground again, which switches the gain boost out.

Excellent intel. Did you remove the Fetrons and replace them with 1.5K resistors?

That jack in the reverb spot may be a padded signal to the phase inverter for a parallel effects loop. If I keep guessing, one of them has to be right.

We don't get too many IIA's around here, so this is actually interesting.

 

[kaluablue]

... Did you remove the Fetrons and replace them with 1.5K resistors? ...

Eeh :shock: Fetrons, plural? As far as I know, the Fetron was a substitute for V1 . There is a switch on the bottom of the chassis '12AX7 / Fetron', which switches to correct anode Voltage for either device. I always used 12AX7 ;-)

Or do you mean the TCRs in the cathode circuits of V1, in the place of cathode resistors? Then, they are still in my amp. Why should I replace them?

There is a good (as far as I can tell) schematic on www.schematicheaven.com//index_HTML.htm
It is identical to the one in the tube ampbook, but magnified.

The 'reverb' jack on obergfey's photos just looks like the original 'send' jack, a mono jack with switch (with a 22k resistor parallel to the switching contact).

BB, your are right, it is in the path from mastervolume to pase inverter, but it opens the circuit if a plug is inserted. It's a serial loop, despite the 22k ?!?

The return jack (PreAmp out /PwrAmp in) is directly connected to the phase inverter, through a huge Orange Drop Cap (~100n?)

 

[Boogiebabies]

... Did you remove the Fetrons and replace them with 1.5K resistors? ...

Eeh :shock: Fetrons, plural? As far as I know, the Fetron was a substitute for V1 . There is a switch on the bottom of the chassis '12AX7 / Fetron', which switches to correct anode Voltage for either device. I always used 12AX7 ;-)

Or do you mean the TCRs in the cathode circuits of V1, in the place of cathode resistors? Then, they are still in my amp. Why should I replace them?

There is a good (as far as I can tell) schematic on www.schematicheaven.com//index_HTML.htm
It is identical to the one in the tube ampbook, but magnified.

The 'reverb' jack on obergfey's photos just looks like the original 'send' jack, a mono jack with switch (with a 22k resistor parallel to the switching contact).

BB, your are right, it is in the path from mastervolume to pase inverter, but it opens the circuit if a plug is inserted. It's a serial loop, despite the 22k ?!?

The return jack (PreAmp out /PwrAmp in) is directly connected to the phase inverter, through a huge Orange Drop Cap (~100n?)

Yes, Fetron singular. The two TCR's are replaced on the cathode with a typical 1.5K resistor. A return to a typical cathode circuit on the MK I, IIA and IIB. It's an official Mesa modification from the MK I.

I photo shopped the pictures to lighten them up and I see exactly what you described. It's much clearer when you can see the wires. I appreciate your input. I've never worked on a IIA, so it's excellent info.

 

[obergfey]

There's nothing better than experience, so I tried and voilà!! the reverb jack is part of the fxloop. It's funny I feel that the more I play it the better it sounds! The amp itself sounds really dark, especially in the lead mode... the speaker doesn't look original, so what could I use to brighten things a bit? Thanks

 

[Boogiebabies]

An EV12L.

 

[kaluablue]

EVM12L of course ... I used to like the VS12 but only at low volumes. Celestion G12H100 is great too.

BoogieBabies, thanks for your words. I'm actually trying to mod my MkIIa to a little more versatility. I can dial in great lead tones (pull treble shift) and powerfull cleans , but only one at a time.
There are some issues in the circuit that make it dark sounding.
The pull bright on leadmaster actually removes a .002 cap from signal to ground right at the input of the drive stage. This cap cuts highs. I removed it and instead installed a 2.2u cathode cap (there was none) to the drive stage. So in normal mode I have the highs of previously 'pull bright' and switched I get (a little) more gain and more highs in the lead sound. This again allows me to lower treble in the tone stage so I get clean and lead more balanced.

obergfey, you can run the presence control quite high, this gives you lots of highs. As on any (Mk) Boogie Volume1, Treble and Presence shape your sound. The pull switches are very powerfull aswell.
Compared to my MkIII the MkII is very midrangy. It's not capable of those shimmering funky cleans, but on the other hand its clean sound cuts through any mix. The lead does so anyhow ;-)

 

[Boogiebabies]

I love revoicing amps too. So a 2.2uf on the 3.3K cathode is your voicing. I just modded a Fender Blues Deluxe with an extra Boogie gain stage. I like to start with two on/off/on toggles loaded with 1uf, 2.2uf, 15uf and .22uf so I can listen to each voicing at the flip of a switch. On the Fender, I went with the C+ style .22uf " Pull Bright". With a 680K grid load resistor and a 1.5K cathode it went from mild to sweep arpeggio, shred wild.

On the IIA, the Fender like 22uf cathode caps must make for a very fat sounding amp, but I don't know why they used the current limiters instead of resistors on the V1. Then, a cathode follower off the V2B ? They claimed to use the Fetron due to the poor quality of tubes, then they throw 260V to a cathode.

Thanks again for the IIA clinic.