6G6 Bassman Output Transformer

[RightLurker]

I'm working on a 1962 6G6 Bassman, and just noticed that the OT is part number 45548, which is the OT used in a 6G8 Twin. I think a 45548 is an 8 ohm OT. The Bassman cabinet is 2X12 and is 4 ohms - 2 x 8 ohm Oxfords wired in parallel. (Most 6G6 Bassmans had single 12" cabinets.) I'm pretty sure the OT is original to the amp - the part number/date code is 606 120 - the twentieth week of 1961. The speakers are original 8 ohm Oxfords.

I've never been able to get the amp to sound right. Could the OT be a problem?

Thanks.

RL

[tubeswell]

Since it uses 2 x 5881s/6L6s, I'd use 4k (or 4k2), but anything between about 2k5 and 6k6 would work with 3k2 - 5k being more optimal. 8k won't harm the tubes too badly, but it won't sound right.

[martin manning]

You're not the only one. Similar story here:

http://www.fenderforum.com/forum.html?d ... ber=682764

Have you tried it through an 8-ohm cab?

MPM

[RightLurker]

Thanks for the quick responses! I'm going to try it with an 8 ohm speaker later in the week. From what I read in Martin's link to the Fender Forum, it sounds as if Fender often used whatever was handy and would work when building amps back then.

[RightLurker]

I think I've made a mistake (imagine that!). I am really new at this.

Is the following statement correct?

If you run four 6L6s through the 45548 OT, it "wants to see" 4 ohms, but if you run only two 6L6s through the same OT, it "wants to see" 8 ohms.

Sorry if I'm confusing things or making no sense.

RL

[martin manning]

Well, sort of. It's the tubes that want to see a particular load, not the transformer; it's just the middleman.

A transformer is like a fixed ratio gearbox, which, at a given power trades torque for rpm. If you reduce the input power by half, you must reduce torque, or speed, or both at the output accordingly. Electrically it's voltage and current that are being exchanged.

In a Twin you have four 6L6's driving a 4-ohm load through this transformer. Suppose you pull two of the tubes and halve the power input at the primary, basically delivering half the current at the same voltage. The voltage on the secondary will be the same as before since the transformer's voltage ratio is fixed, but the current and therefore the power delivered will be cut in half: power = current x voltage. However, if you still have a 4-ohm load connected, the power demanded will be the same as with four tubes because both the voltage and the load resistance are the same.

What actually happens is the voltage-current relationship on the primary must adapt to the secondary load, and the power tubes are forced to operate on a different part of their characteristic. To halve the power demand on the secondary, you double the load resistance to 8 ohms (reducing the current by half, since current = voltage/resistance), and the two remaining tubes will then see the same load as before.

Another way to say all this is that the impedance ratio of the transformer is fixed. In this case, it’s around 500:1. For typical operation, two 6L6’s need an impedance (a voltage-current relationship) of about 4,000 ohms, and the transformer will match that requirement if an 8-ohm speaker is connected to the secondary: 4,000/500 = 8. If you parallel two more tubes on the primary side the impedance needed is reduced by half, to 2,000 ohms. The secondary should now be connected to a load of 2,000/500 = 4 ohms.

An interesting question is, what is this doing to the feedback loop? Is the value of the feedback resistor 100K like on the 6G6 schematic? PI tail resistor 4K7? If these are unchanged, the negative feedback and presence function are something other than what they should be. Even if you connect to the appropriate 8 ohm load, I think the feedback voltage would be larger than intended by 1.4x.

MPM

[Structo]

Great explanation Martin.

I always have trouble remembering which way to go on the impedance when pulling tubes.

[RightLurker]

Martin -

That's the best explanation I've ever read about impedance, etc. Really great. And you zeroed in on one of the amp's chronic problem areas - the presence control has never done much of anything, whereas on other Fenders of that era the presence control has a very pronounced effect. I'll check the resistor values you mentioned, but I've been studying the amp against the 6G6 and 6G6A schematics and layout diagrams for a couple of months, and I think I identified just about every component and matched it with what is in the schematics.

Thanks again.

RL

[martin manning]

Thanks, guys =) Maybe the quickest and least intrusive way to set things right is to replace the OT with the correct spec. Plugging in to an 8-ohm load will give you an idea of the difference this will have, but it won't be the whole story re the presence. Funny, googling around for the Twin transformer you have, I found someone who said they had bought a used one, "pulled from a blonde Bassman."

MPM

[RightLurker]

Martin -

Thanks for all this. I was wondering if changing out the OT might be the best thing to do. If so, would it be best to look for the correct OT on the used market, or see if something new from Mercury Magnetics might work?

Thanks again.

RL

[Structo]

If it were me I would get a OT that had the correct impedance on the primary and has a secondary of at least a 4 & 8 ohm taps.

I like OT's that have the 4, 8, 16 ohm taps, more versatility.

Check out Magnetic Components.
They are very reasonably priced and I haven't heard one bad word about their quality or dependability.

Here is a 50-60 watt OT with 4, 8, 16 ohm sec.

http://classictone.net/40-18001.html

[RightLurker]

Thanks, Tom. It's beginning to sink in that if this amp is ever going to be "right," the OT is going to have to be replaced. I hate the idea of removing transformers from a '62 Fender amp, but if they were wrong from the factory, what are you gonna do? I'd be comfortable having a go at it myself with a "drop in" replacement OT, but I wouldn't have a clue how to handle a multi-tap OT. This is beginning to look like a job for a pro.

Again, thanks.

RL

[Structo]

OK, yeah I see your point about a multi tap OT.

Fender did some strange stuff. Usually rather than slow or stop production Leo would have them substitute values or parts to keep the line moving.

The tube charts are a good example of this.
Just how expensive was it to make new charts for a new circuit?

[RightLurker]

Tom -

You're right about the way Fender did things. My amp is what I guess you would call a transition model, because it has a tube rectifier and the tube chart says 6G6, but the preamp circuit is like a 6G6A, and it has two 12" speakers instead of one.

Your point about versatility is well taken, but this amp is not likely to leave my house and will be used only with its own speaker cabinet. The fact is, when (and if) I get it straightened out, it won't see that much use.

Thanks again.

RL

[martin manning]

My thought is don't change more than is necessary. A drop in replacement OT would be correct in appearance, require no additional mods to the amp, and you can keep the (now) matching 2x12 just as it is. Magnetic Components makes a 4-ohm standard version http://classictone.net/40-18011.html , and there is the Hammond 1750L http://www.hammondmfg.com/guitarLineOT.htm . Both of these have the core raised so wires can pass underneath. I'm sure there are Mercury and Heyboer units that would work too.

MPM