Output Transformer Placement

[Smitty]

I'm designing a new amp and would love to get some input. The design is sort of Bassman chassis in a Princeton cabinet. Yeah, I know, I amaze myself sometimes. The reason is that guys are looking for a grab and go amp that'll put 35 watts of power into a 12.

Well, I'm down to where to place the output transformer. As a Blackface Fender guy I've always just chucked them next to the choke so the wires would be close to the output tubes. Problem is that in a tiny cabinet that puts the OT right on top of the speaker magnet. So now I'm thinking that I want to try it on the input side of the chassis so I can get it away from the speaker.

I've got a few questions:

1) Where would you run the center tap and plate leads?

2) Given that they are long runs would you still twist them together?

3) Would you shield them?

4) What am I missing?

I'm still going to tinker with this once I get my prototype chassis, but I'd like to try as many practical suggestions as possible.

Thanks everyone!

Smitty

[tubeswell]

Twist the plate leads and run them together, with the CT, on the 'outside' of the chassis to a point where you're adjacent to where the output tube sockets are on the 'inside' of the chassis (that's how I do it and it works for me). Try and keep the OT as far as practicable away from being too near the input jack. (Altho' a steel chassis will provide some shielding). (I guess you know about keeping the tranny cores at right-angles to each other)

[ETR]

I've never had a problem with that with my builds and I always put the OT on the opposite side of the PT for balance if nothing else. Sometimes they're very close to the input jack and I expect problems but never have any even with high gain. I do think lead dress is very important tho and the suggestion of keeping it outside and away is a good one. I do think the actual fields generated by the coils of the OT (if its not shielded, and most aren't) are more worrysome tho.

[Chuck H]

OT on the input side is the standard layout for Soldano. I think those qualify as high gain amps. So thats at least proof that it can be done successfully.

I have never done it. But I did do some re-design work on an amp that was layed out this way. I remember thinking that it was a bad idea. But there was no problem whatever associated with the OT being right under the input side of the preamp. As was already mentioned, and as I witnessed on this amp, run your OT leads on the outside of the chassis to enter at a point close to where they will be soldered. Also, this was a steel chassis. I am not 100% on this but as was mentioned, it seems like steel would provide better electro magnetic shielding that aluminum.

[MWJB]

How about having the OT next to the PT and running the choke out to the input end of the amp? Will that give you adequate speaker clearance?

[R.G.]

... guys are looking for a grab and go amp that'll put 35 watts of power into a 12.

The guys might want to look at the Visual Sound Workhorse "Pony" amplifier then.

Smaller than a Princeton, more solidly built; 30W from a pair of 6L6s for reliability, single Celestion. Designed for both reliability and easy maintenance.

[BluesHawk]

Lookin for a Pony ride

The guys might want to look at the Visual Sound Workhorse "Pony" amplifier then.

Smaller than a Princeton, more solidly built; 30W from a pair of 6L6s for reliability, single Celestion. Designed for both reliability and easy maintenance.

I'm interested in the Pony. I play Blues and favor what I imagine as a 1950s West Memphis tone...thick, a bit dirty. My 1998 Danelectro Honeytone (Nifty Fifty) does it for me.

I first learned about the Pony from some Blues guitarists whose style is close to mine; they recommend it. Just wonder if you agree it has gain or distortion that can thicken & dirty things up a bit?

And if so, can you suggest any leads on an NOS or decent used Pony?

Thanks,

Doug

[olddawg]

Many older amps had the OT mounted on the speaker frame, even PP 6v6 amps. Some modern frames still are manufactured with the mounting area and holes stamped into them. My point is. If you are worrying about being close to the speaker causing a problem, it probably isn't an issue as long as it isn't grinding on the speaker magnet or frame. I've also seen OTs mounted to the wood on the side of the cabinet many times. Especially if the speaker has been replaced with one that had no place to mount the original OT.

[printer2]

Do the headphone looking for the least hum thing. I was laying out a chassis today and I had hum pickup even when turning the OT and the only practical mounting was on the opposite end of the chassis.

[TarheelTechinTraining]

Hey printer not to ask a noob question or highjack the thread. But what exactly is a " headphone looking for the least hum thing"?

[printer2]

Hook up your headphones up to the output of the OT, then with power to the PT you move them around to get the lowest hum. The transformers can couple the 60 Hz AC so even if you do not have any tubes in the amp you still hear hum coming from the OT.

[TarheelTechinTraining]

I get it. I once say a fellow find a static/hum in a car stereo. With the car running the ignition could be heard through the stereo. He used a transistor radio and a tapehead recorder. Basically the same idea. Found out it was the ignition wires, non- resistors.