Output Transformer for a Bassman

[MysteryFever]

Hello everyone, I need a bit of help if anyone is willing. I've got a 66' Bassman with farty low end (what else is new?). Everything in the signal path has been changed/replaced in the interest of resolving this except the OT. Now I find that these suckers can have a significant impact on the overall tone and frequency response of any given amp. There are a lot of transformers out there with ambiguous names... (like A431S for example, an OT with fabulous low end but almost assured incompatibility with my amp).

Is anybody willing to share a recommendation or some insight on finding a compatible OT with good overall freq. response? Personal experience perhaps? Much appreciation to anyone with some input.

[Structo]

Which circuit is your amp?

Is it a AB165?

Well it is a bass amp but it never was very good at that was it?

I think if it were me I would reduce the value of the cathode bypass caps on V1 to start with.

You can try a 10uf but I would probably try a 5uf first.
Probably do the same on V2 if the bass is still heavy.

Then if there is still too much I would start on the coupling caps.


I'm sure there is somebody here that is more into the Bassman amp that can tell you for sure what to do.

Good luck!

[rdjones]

Nothing at all ambiguous about the A431, it's the same highly regarded Dynaco UL output in use since the '50s, albeit in modern 'cloned' form.
Just had a Sunn through here with one, when pushed it would get a bit wooly without being farty, if that makes sense. (I usually try to avoid 'buzzwords')

There are two ways to approach voicing low end response.
The first and most common is to limit low frequencies so they don't distort, as Structo has started to detail.
The other, and opposite, is to increase low end capability so the frequencies are reproduced cleanly.
You can also combine the techniques so that lows passing through the higher level preamp stages is reduced there to avoid overload, but allow more low end in the output stage by, for example, using a more capable transformer like the (cloned) Dynaco.

The impedances of the A431 should work quite acceptably in the Bassman.
The mounting centers are different (?) so you might need to drill a few new holes ...
Oh, the screen taps can be ignored if being used for guitar duty as is common with UL outputs used in guitar amps.
The big secret here is that UL helps clean up low end distortion but that's mainly in the lowest octave i.e. for bass guitar.
This is for guitar and not bass, correct ?

RedDog

[billyz]

Try changing the coupling cap feeding the phase invertor to 500pf instead of .1mf . That should clear things up. If you still need more clarity change the caps from the phase invertor to the power tubes to .02 instead of .1mf .

[billyz]

If your amp is an ab165 circuit there are a few more things you should do too. like get rid of the 220k feedback resistors around the power tubes back to the PI, Redo the bias circuit and PI plate loads. Basically mod it to a aa864.

[Cliff Schecht]

Also it is common to increase the B+ capacitance, especially at the first post-rectifier capacitor, to tighten up the low-end response. Makes things tight and punchy.

[Roe]

up filtering at mains and even the screens. add one filter cap per preamp tube, do not use the cheap fender way where a lot of tubes share one cap. decrease coupling caps a little to remove the low bass, and your bass should be tight. High voltages are also preferable. I have two twins built this the way - they are tight and punchy as hell.

also, try real 7581As

[MysteryFever]

Thanks all of you for helpful and informative replies! It was an AB165 but has been heavily modified as follows: Baxandall-type tone stack for Bass channel and normal channel has been tweaked around a bit, an overall decrease in coupling capacitor values (although I had not considered bringing down the PI coupler to 500pf, it's at .001 right now I believe), removal of the 220k feedback resistors in the power section, conversion of the hum balance to a real bias control, converting the NFB to a more classic fender scheme, switching the PI plate load resistors to the 100k/82k, and modding the power tube sockets to accept EL34's (which are currently installed and working fine).

Structo, your recommendation to decrease cathode bypass cap values seems to tie in with Roe's to replace the "double cap" fender method. I will most certainly be trying this with lower values.

RedDog, yes it is for use as a guitar amp. I have another question for you; if I were to replace the OT with a direct drop in replica from hammond or perhaps this one: http://www.triodeelectronics.com/surebaco59ba.html, would this help with my problem, or would I just have a new transformer with the same problem?

Cliff Schecht, are you referring to the filter caps under the chassis when you say "B+ capacitance"? Also, is that the same as what Roe is saying about upping filtering at the mains and screens?

Again, thanks to all of you for being helpful!

[billyz]

If you want to increase the filter capacitance. First go with series 220uf @350vdc instead of the stock 70uf @350vdc in series at the first filter before the choke. This will give you 110uf @ 700vdc. Next change the cap right after the choke to 40uf @ 500vdc instead of the 20uf. I would leave the rest as stock. The only reason you can go this high is because you have SS diodes and not a tube rectifier.
If you get motor boating then you went too high. But I have used these values before. You might still want the 500pf input cap if don't plan to use it for bass.

[rdjones]

RedDog, yes it is for use as a guitar amp. I have another question for you; if I were to replace the OT with a direct drop in replica from hammond or perhaps this one: http://www.triodeelectronics.com/surebaco59ba.html, would this help with my problem, or would I just have a new transformer with the same problem?

The difference with that transformer is the additional windings for the 8 Ohm tap. Otherwise a very close copy.
So unless your problem is a mismatch (I doubt that it is) it will be the same.

In a more general sense I find it interesting that the Bax tone circuit was used.
Is the sound better on one channel more than the other ?

The AB615 has an extra preamp filter that the earlier designs lacked, but it is still shared by more than one tube.
It's marked 8u on the schem but 16u is usually found there.
Not much to be gained here by going past 20-30u.

rd

[jaysg]

Blackface farting is often caused by blocking distortion at the output tube grids. Reduce the 0.1uF couplers between the PI output and the grids. Try .047uF, .033uF, .022uF, 0.01uF...use the largest ones you can.

hth, ymmv

[rdjones]

Or up the 1.5k grid stopppers to as high as 4.7k.

Kinda why I was curious about any channel to channel difference.

rd

[MysteryFever]

RedDog, the bax was used as an experiment that yielded very useful results. Of the two channels, this is the one that is usable atm because the bass is not as thunderous on this channel. It does sound a little sterile however, not as aggressive as the other, which has a high mid bite to it and absolutely earth-shaking low end. It's a plate-following tone stack with a 500pf treble cap, 100k slope resistor, and .022uf bass and mid caps. If I could roll off some of the bass in this channel without neutering it, it would be a great-sounding channel.
What will increasing the grid-stopped value accomplish? That's one I haven't heard before. Also to jaysg, I believe those are at .022 now. I could try a smaller value, I've got plenty of orange drops lying around...

[rdjones]

Merlin goes into some detail about grid stoppers and it's a great place to mod for tone and distortion shaping.
It reduces the grid current that flows when a tube is driven into clipping.
It also forms a pole of high frequency rolloff based on the resistance and the input capacitance of the tube, and is usually way outside the audible range.

If the problem is only when playing at volume (ie clipping) then it probably is blocking distortion, and likely due to bass boost in the channel.

What about the NFB ? Is it the normal 820/47 ?
I know the AB165 is different than the typical Fender.\

rd

[Roe]

RedDog, yes it is for use as a guitar amp. I have another question for you; if I were to replace the OT with a direct drop in replica from hammond or perhaps this one: http://www.triodeelectronics.com/surebaco59ba.html, would this help with my problem, or would I just have a new transformer with the same problem?

The difference with that transformer is the additional windings for the 8 Ohm tap. Otherwise a very close copy.
So unless your problem is a mismatch (I doubt that it is) it will be the same.

In a more general sense I find it interesting that the Bax tone circuit was used.
Is the sound better on one channel more than the other ?

The AB615 has an extra preamp filter that the earlier designs lacked, but it is still shared by more than one tube.
It's marked 8u on the schem but 16u is usually found there.
Not much to be gained here by going past 20-30u.

rd

yes, 20uf is sufficient if its used by one tube only.