Disassembly and Retubing

MA Heads and Combos

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SteveD
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Re: Disassembly and Retubing

Postby SteveD » 24 Jul 2010, 05:59

Nice shots Dale. :Thumbs

BTW, Don't throw the old power valves away because they are also Svetlana winged C. :winking
Still my guitar gently weeps

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Re: Disassembly and Retubing

Postby Nebulozny » 26 Jul 2010, 01:42

So, Dale just changed tubes to the same brand ? I didn't know those stock tubes are winged C. Are there any differences in those marshall branded and tubes that are sold under -winged - C name?
Steve, don't tell me preamp tubes are Tung Sol cause I just installed those in my MA :bgrin

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Re: Disassembly and Retubing

Postby SteveD » 26 Jul 2010, 12:24

There's no physical difference, we just have them badged as do most companies who use them in mass production.

No the preamp valves are not Tung Sols.
Still my guitar gently weeps

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Re: Disassembly and Retubing

Postby Lizzy » 08 Nov 2011, 11:22

A couple more tips for those wanting to bias their MA100H.

As SteveD mentioned you must remove the chassis from the cabinet and then turn it upside down to access the bias
test points and trim pots. I found the putting the empty cabinet face down on a padded surface and then putting
the amp chassis back in it with underside facing upwards worked very to support the amp in place while I was
working on it.

The 100 watt MA's have 2 trim pots marked Trim 1 and Trim 2 (as shown in picture posted by Dale). Trim 1 for the
top pin test point and Trim 2 is for the lower pin test point. This is where you may encounter difficulty unless
you have some prior experience with biasing amps with 2 pots. Each trim pot works against the other one, so as you adjust 1 pot up or down the other test point will go the opposite direction down or up.

For example if you are measuring on the top test point pin and turning the trim 1 pot up the lower test pin
mVDC/mA readings will fall at the same time . So you then have to adjust the trim 2 pot up which in turn also
brings the trim 1 readings down. It's also worth noting that the trim pots are extremely sensitive so it only a
very very slight turn to make adjustments.

So this could become frustrating really quick for a novice as you can end up jumping back and forth between trim
pots trying to bring one side over/under 76mvdc so you have room to adjust the other trim pot up/down to bring
both sides as close together as you can.

I have biased other amps with single trim pots however this was my first attempt at biasing with 2 trim pots. Fortunately I had just recently watched Bob at Eurotubes video on biasing a Marshall DSL100 where he talks about the same issue of each trim pot working against the other so I kinda knew to what to expect going into it.

I had taken my chassis out of the cabinet just to swap the V1 tube but thought I'd check the bias since I had it apart anyway. I found from the factory and after 6 months of use it was reading 72mVDC on one side and 73mVDC on the other. The best I could manage was getting it to 75.x on each side before I got frustrated with trying to get both sides to stay in sync. Not sure how much of a real difference 1 mVDC makes in terms of performance of the tubes. Anyone?

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Re: Disassembly and Retubing

Postby Crunchifyable » 08 Nov 2011, 14:00

I wouldn't sweat it. The best differential I can manage is .5 mV on the 50watter, and even then I'd imagine they separate even further in time by a few mV.

I think it's fine considering these are idle readings, which basically go out the window once a signal is applied.

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Re: Disassembly and Retubing

Postby washburn 1 » 08 Nov 2011, 14:55

:howdy Hi Lizzy
You say that when adjusting bias on the100H that the trim pots work against each other. I have the 50H but it also has two trim pots,one for each valve.So could this be the case for this amp also or will it strictly be one for each valve and they will NOT interact with each other.Just curious as mine seemed to bias very easily and accurately. Btw am currently running TAD el34b-str AND THEY SOUND WICKED!!! Sound is now much more open and the amp sounds as if it can `breath` properly, they really have brought life to the amp as if a towel has been removed from the front of the cab. Cheers, Kev. :jam

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Re: Disassembly and Retubing

Postby Lizzy » 09 Nov 2011, 15:53

Hi Washburn, I don't know how it works out on the 50 watt presumably where you have 1 trim pot/per valve versus 1 trim pot /per 2 valves on the 100 watt. Though I'm kinda wishing myself I had stuck with a 50 watt version for all the conveniences it brings only having 2 power valves but the 100 watt was the only thing in stock locally when I bought it. I suppose on your 50 watt you could also easily pop in 2 completely unmatched/different brand valves since you have a seperate trim pot for each where as on the 100 I still have to have at least 2 matching pairs.

Here's the link to the Eurotubes video if anyone wanted to see it:

http://www.eurotubes.com/euro-Marshall-DSLTSLV.htm

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Re: Disassembly and Retubing

Postby Phano36 » 10 Nov 2011, 03:30

It's a good point what Lizzy is brining up !
note that I biassed my MA100C a couple of times. Like in the Eurotube the MA100 contains 2 pairs of EL34, but I didn't notice any changes in the voltages. I will pay more attention the next time ! Or maybe it's just the way that the amp is design ?
Guess that we need Steve D input on this !

And Dale like Steve D suggessed dont through away the original tubes ... Wigged " C " price went up since last year... they used to be around $38,00 the pair now they are $ 74,00 the pairs. And If you dont want to keep then anyway... I will give you my adress : )

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Re: Disassembly and Retubing

Postby SteveD » 10 Nov 2011, 07:14

The two trim pots are interactive to a degree and I would recommend setting them both in the central position to start with and making small adjustments from there in whichever direction required. It's a balancing act - adjust one trim, then the other, then back to the first, then back to the other etc.
Still my guitar gently weeps

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Re: Disassembly and Retubing

Postby rayce » 24 Jul 2012, 18:07

Wanted to say, sounds like setting up the bias is alot like tuning a FR style floating bridge. While I really like to fiddle around, my son n law doesn't have the same opinion when he tries to Drop D tune it. LOL :cowbell

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Re: Disassembly and Retubing

Postby surfnorthwest » 24 Jul 2012, 19:29

rayce wrote:Wanted to say, sounds like setting up the bias is alot like tuning a FR style floating bridge. While I really like to fiddle around, my son n law doesn't have the same opinion when he tries to Drop D tune it. LOL :cowbell
Except the FR doesn't pack the voltage to kill you. :eek Welcome to the Forum. :welcome
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Re: Disassembly and Retubing

Postby rayce » 25 Jul 2012, 20:27

Thanks for the welcome and I definitely noted the dangerous voltage. I just set up my MA100H but had a much different reading from the bias port with JJ EL34's.

I have a question if not imposing too much.
Will it make a difference if you have a 16 Ohm speaker load vs. 8 Ohm? I ask because the boxes that the JJ's came in had 15/38 on them and I set them up for 15mA and switched to mVDC to see it was reading 38mVDC. Just a guess but if I went with an 8 Ohm load would it double the 38mVDC which would be the 76mVDC setting.

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Re: Disassembly and Retubing

Postby thephantum » 26 Jul 2012, 11:43

As long as you match the impedance by having the speakers plugged into the correct jack, it will not make a difference on the bias setting.

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Re: Disassembly and Retubing

Postby Lizzy » 26 Jul 2012, 17:47

Welcome Rayce, I noticed you said you are getting a reading of 38mvDC at the "bias port". I assume you are using an inline bias probe on 1 tube and not the test points on the amp? The 2 onboard test points on a MA100h measure each of the 2 pairs of tubes so that is why the reading is double, ie 76mvdc.

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Re: Disassembly and Retubing

Postby rayce » 26 Jul 2012, 19:37

Thanks for the response Lizzy,

I am reading from each of the outer pins of the factory test port on circuit board to common ground. Just as the image shows in this thread. I was a little surprised to see such a different reading and I am using a Fluke77 meter. My plate voltage is 470VDC.

FWIW, I pretend to be an electrical engineer for a living so I'm reasonably sure I am doing everything correctly.

MMM.... :help

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