Marshall 4212 OT Question

[Wilder Amplification]

OK so I got a Marshall 4212 (2x12 combo version of a 2205) that was converted to a 1987 circuit by a complete beginner. As luck would have it...the guy had V5's plate wire and the CT reversed. Now I've seen guys get the plate wires themselves backwards, which gives the SBOD scream, but a plate and CT wire!? WTF?

So...fixed that. Tested the amp...with the PPIMV past about 6 you'd get a loud hum and squeal. Shut it off...upon further investigation found that the speaker jacks weren't grounded.

So..grounded the speaker jacks...now all is well.

Now, this is a 6550 Marshall. The guy who worked on it prior to me installed a Mercury Magnetics O50JM-MM, which Mercury advertises as their "6550/KT88/6L6" OT. Well, to make sure the guy got the secondary taps in the right location I injected wall voltage into the primary with the CT disconnected from the supply, and found that he in fact had the 4 and 8 ohm taps reversed. Fixed that...but then I calculated an impedance ratio of 400:1 on the 16 ohm tap (6.4K:16).

I've never seen that high of a plate load ran on a 6550 amp. Moreover, the data sheet spec for dual rail operation at 600 volts only calls for 5K, UL operation calls for 4K, and standard "Class AB1, Tetrode Connection, Fixed Bias" operation calls for 3.5K Zp-p @ Va = 450/Vg2 = 310.

Why Mercury would wind an OT with that kind of a plate load and advertise it as a 6550 OT is beyond me since the data sheets even at 600 volts call for something much lower than that.

So...having sad that, I'm worried that the screens will get overtaxed if I leave it in there. The way I see it, I have two options -

1) Tell the customer that the guy originally installed the wrong OT. See if Mercury will exchange it (which is more than likely a no-go because the OT was purchased 6 months ago and it took the guy that long to get the amp back). Or...

2) Inform the customer that it is in fact the incorrect OT, but to treat his 4/8/16 ohm taps as 2/4/8 ohm taps, which will reflect a load much closer to what these things wanna see at the voltage they're running at.

Which would be the best option here? The customer is a really nice guy and is already ballz deep in $$$ on this whole thing due to the history he had with the other guy who worked on it and when I send this thing back to him I wanna send it back with 100% confidence that everything in this amp is good to go.

[Chuck H]

What's the primary Z on the 4 and 8 ohm taps?

If they are equally askew just tell the guy the real story. The OT MFG built this as a universal replacement and to be ideal with his tubes he should operate the amp with the impedance switch set to twice his load. Or relabel the impedance switch somehow so there's no confusion. The problem I see is that in a 2x12 combo it may be wired for 16 ohms (which is easy enough to change) or he may want to plug it into a 16 ohm cab sometime as this is how most 4x12 cabs are wired.

Chuck

[Wilder Amplification]

What's the primary Z on the 4 and 8 ohm taps?

If they are equally askew just tell the guy the real story. The OT MFG built this as a universal replacement and to be ideal with his tubes he should operate the amp with the impedance switch set to twice his load. Or relabel the impedance switch somehow so there's no confusion. The problem I see is that in a 2x12 combo it may be wired for 16 ohms (which is easy enough to change) or he may want to plug it into a 16 ohm cab sometime as this is how most 4x12 cabs are wired.

Chuck

The Z is the same on all taps...6.4K Zp-p. Obviously the ratio itself is different on all taps but the actual reflected Z is the same.

That's what I thought...tell him to treat the settings like 2/4/8 ohm settings, but there again you no longer have the flexibility to run a 16 ohm cab, which is what is telling me to let him know that it's the wrong OT.

Universal or not, 6.4K is still way too high for 6550s running at 440-450 volts as per the data sheets.

[Chuck H]

Universal or not, 6.4K is still way too high for 6550s running at 440-450 volts as per the data sheets.

I agree. But MM decided that was OK so it's thier call on that end. What should you do???

which is what is telling me to let him know that it's the wrong OT.

But it's not. It's just the way MM intended it to be. I'd give him the real story and your "recommendations" for it's use. That way you and MM have both done what you think best. It's up to the owner at that point. Why make it hard for yourself? By telling him it's the wrong OT you could open a can of worms with counterpoint on conversations he may have calling MM for tech support or some other scenario.

JM2C

Chuck

[Steve Conner]

I don't think it's going to be the end of the world... I've run 6550s on a 6.6k OT before and nothing bad happened. Well not yet.

They may well sound better loaded down to 3.3k though, why not try it and see?

Or out of curiosity, you could try overdriving them into a 6.6k load and see if you can see any screen glow.

[Wilder Amplification]

I don't think it's going to be the end of the world... I've run 6550s on a 6.6k OT before and nothing bad happened. Well not yet.

They may well sound better loaded down to 3.3k though, why not try it and see?

Or out of curiosity, you could try overdriving them into a 6.6k load and see if you can see any screen glow.

Very true Steve...I'll probably do just that.

More than likely better to just let the customer know that there is in fact some room to mismatch since they wound it with a higher impedance. MM's ideal load is 6.4K, but datasheets state that they'll run into a 3K-3.5K at that plate voltage so it gives him some "wiggle room".

[Chuck H]

I don't think it's going to be the end of the world... I've run 6550s on a 6.6k OT before and nothing bad happened. Well not yet.

Come to think of it, I have too. Before I started building amps I had a Marshall MKIII (pair of 6550's) that I ran into a 16 ohm cab off the 8 ohm tap. I thought it sounded better that way That has to be close to or even greater than 6.6k. I never had a failure. But I didn't use the 6550's long before starting my amp mod/electronics adventure on that very amp by modding it to use EL34's.

Chuck