changing the OT for fewer watts of output

[jdesille]

Hello. I’m wondering if anyone can help me figure out how low I could go in terms of output transformers, specifically with reference to the Hammond product line, if I wanted to lower the output of my recently-completed Two-Stroke. It currently has a Hammond 125ESE, which is rated at 15W with a Max DC bias of 80ma, but after having played it in my apartment, where it sounds very loud (not a bad thing in itself), I have been considering smaller Hammond OT, or even some Fender Champ replacement OTs, which are rated at 5W; the Hammond 1750C and 125BSE come to mind. The amp is configured with a single 6V6 output tube.

However, I don’t know enough yet to figure out if the transformers’ different “Max DC bias” ratings are a crucial factor here. In other words, the small 125ESE has a max DC Bias of 80 mA, and its smaller cousin, the 125BSE, has a max DC bias of 45mA (I assume the 1750C has a similar rating).

My questions, then, are: What would happen if I replaced the Hammon 125ESE currently in my Two-Stroke with a smaller Hammond like the 5W 125BSE? What about the much cheaper 1750C? Should I expect trouble? And what does “Max DC bias” have to do with it?

Thanks for your patience in reading this. It’s my first post on any forum ever. I have attached Hammond transformer specs for 1750C (assuming that most folks here are familiar with specs of 125ESE and 125BSE).

[Robin]

Why not build an attenuator and use it with a 6V6, you’d get low output from the speaker and as long as you limit the attenuation to about 12 db you’ll still get the cool 2Stroke tone. Gerald Weber wrote an article in Vintage Guitar Magazine some years about on how to build a 6db attenuator and then again some time later a slightly fancier version with a switch to select 6 or 12 db of attenuation. I build both, they were both super easy to build and work great. There is a “how-to” tutorial on building the two stage version in Weber’s “All About Vacuum Tube Guitar Amplifiers” book too. There are probably other DIY attenuators online.
Probably cheaper than a new transformer and would give you more output options.

[Andy]

I would second the Attenuator option. I use a Weber MASS to test amps out at higher volumes when the family is around

There are other things you can do to reduce the overall gain as well. You can always try a 5751 tube, or even a 12AU7. Of course, that will change your distortion amount as well. You can also change your cathode resistors to a little bigger value to get less gain (this can be a bit tricky as you are changing the tube bias). You could also snap in a resistor in series with the 22n resistor (C6 on the TAN schematic in the wiki). !5K, 33K, 68K would drop the signal going into the power tube.

But to get the same exact-ish sound at lower volumes, then the attenuator is the way to go.

Another option is to get a lower db rated speaker.

[jdesille]

Thank you. The attenuator is a great idea that I hadn’t thought of off hand. Can you think of any drawbacks to using the attenuator? In the meantime, I’ll do some digging online for DIY schematics and layouts.

[jdesille]

Thank you for those possible approaches. The speaker one might be a good short-term fix, as I have a few of them laying around.

On the technical side, and this question would go to both of you guys so far, would the smaller transformer be a burden only in terms of additional cost, or is the Two-Stroke going to lose some of its nice distortion characteristics?

I’m still reading through Rider’s Inside the Vacuum Tube before I try heavier books like Kuehnel’s, so I don’t yet fully understand the interplay between the OT and the power tube. My thinking at first was that a smaller OT transformer, especially one of the same series by Hammond, would produce a similar sound at lower volume. In other words, I suppose I had the idea that it would work like the attenuator. Is that not true? Or is it so true that I ought to just save myself some dough and build an inexpensive attenuator? Thanks again.

[Robin]

Too much attenuation changes the tone and makes it sound boxy. Be sure to use the correct value power (wire-wound) resistors for output impedance and using resistors with a high enough power rating is important but with the low wattage amp like the 2Stroke that won’t be much of an issue. 15watt resistors would do for amps up to 25w according to Gerald Weber’s book. I reviewed my copy and found that construction of both the single stage and two stage versions of his attenuator are featured.

[djroge1]

Good info in this thread so thanks. I need to look into that attenuator idea.

[Author]

Posts