OT impedance and break up relationship

[bob p]

One thing I have not seen brought up: Isn't a Bassman transformer made for bass frequencies or wider bandwidth?

what amp would distort more a Bassman on 10? or a SR on 10?

Interesting question.

I'm not sure about the wider bandwidth thing coming from the OT. The lore definitely tells us that, but I'm not sure that the lore is 100% accurate. Trying to be objective about this and relying on numbers: Which Bassman are you referring to?

The most similar Bassman to compare to a SR is the Bassman 10. It uses the exact same power supply as the Super Reverb. Both are rated 40W at 5% THD and both meet the same specs. The amps have similarly weighty iron in the OT, yet both sound very different because of their different speaker compliments.

A 50W Bassman uses a more stout power supply and a bigger iron everywhere. It is rated for 50W at 5% THD. The Bassman 50 uses essentially the same preamp as the SR (without reverb and tremolo). It's not so much the preamps that are different in those amps, it's really the PS and the output stages that are different. that makes direct comparison difficult.

The 40W power supplies and OT are smaller than the 50W versions, which is to be expected because the 50W amps are producing 25% more power. 50W iron has to be bigger. But based on my experience using them as bass amps, the smaller size of the 40W iron seems to be attributable to their power ratings, not their frequency response. At least that's been my experience comparing the 40W vs. 50W Bassmen as bass amps. Both sets of electronics reach their stated goals.

To the extent that the AB763-type preamp stages all deliver the same signals, it's the power supplies, the output stages and the speakers that are different between those amps, and it seems that the answer would have to lie there. At their rated power all of these amps produce 5% THD. using that as a sort of equivalence assessment, I'm thinking that the differences in bass response largely come from the speakers/enclosures. The Bassmen have been outfitted with speakers that attempt to provide bass response, while the SR is definitely not outfitted with speakers having that same goal. Cabinet swaps suggest that the SR OT seems to be more capable of providing bass response than it's stock speakers are capable of reproducing. With that said, there is a fine line between enough bass response and too much bass response that causes Fender farting, so we commonly meddle with this by downsizing coupling caps in guitar amps.

To make a long answer short, I think that the difference in bass response isn't as attributable to the OT iron as the amp lore would suggest. The iron is sized for power and BW seems to be met in both the 40W and 50W amps. I think bass response is more a function of the speaker compliment. In an amp like the SR the speakers are definitely the weakest link when it comes to bass response. The 40W OT iron seems to be appropriately sized for BW, and with the right speakers/cabinet the smaller 40W OTs do a decent job at bass reproduction.

I wish I had a better answer.

[dstrat]

Thanks , yes after typing that i realized there are a lot of Bassman amps over the years. I really wanted to know if there was a difference in transformers
because I have heard of guitar players doing the Bassman iron in a super. but I think maybe it was tweed Bassman was that they were doing.

[Chuck H]

Thanks , yes after typing that i realized there are a lot of Bassman amps over the years. I really wanted to know if there was a difference in transformers
because I have heard of guitar players doing the Bassman iron in a super. but I think maybe it was tweed Bassman was that they were doing.

The tweed Bassman OT was some pretty good iron from what I've read. Something like seven interleaves and stout. I've never played through one myself. I've played clones, but that can't be considered the same thing. I do feel that SRV's In Step recordings were some of his most bland tones. Though also some of his best playing too. I've read that most of those recordings were done with a stock 5F6A. An iconic amp if there ever was. The reverence for that model defies quantification. But I couldn't hear it with Stevie playing through one and I haven't experienced it for myself. YMMV

[Randall]

Bob P, do you play guitar professionally?

[dstrat]

The tweed Bassman OT was some pretty good iron from what I've read. Something like seven interleaves and stout. I've never played through one myself. I've played clones, but that can't be considered the same thing. I do feel that SRV's In Step recordings were some of his most bland tones. Though also some of his best playing too. I've read that most of those recordings were done with a stock 5F6A. An iconic amp if there ever was. The reverence for that model defies quantification. But I couldn't hear it with Stevie playing through one and I haven't experienced it for myself. YMMV

Thanks Chuck, I have also heard(read) that about the 5f6-a output iron, also never had the pleasure of one personally.Few on Youtube I have heard sound pretty good, but 50+ years on I wonder how they sounded new?
your right the clones likely do not get there. I just wondered if the blackface Bassman iron was created equal or simply off the shelf , what Leo could get is what got used. which leads back to the OP's question. No clear answer I'm afraid as not enough information about Schumacher iron out there.

also agree Instep was Stevie's best playing but maybe the tone got lost in the process? who knows.

[Enzo]

Randall, the question about input was in response to your only 39 watts thing. When you drive an amp through the preamp and run it up to clip, chances are the clip is in the preamp, not the power amp. Thus to truly measure output of the amp, you feed the phase inverter directly so you can run the power amp up to clip.

The reason I suggest looking for breakup is as bob mentioned - knowing if there really is a problem or not. Having the OT off by a few percent is not likely the problem. But if you can go back to the customer and explain that what he is hearing is the normal breakup of a phase inverter clipping (or whatever), it not only points away from OT concerns, plus it directs future efforts. Such future efforts are to either live with what is normal, or to spend a lot of time re-engineering it to eliminate the symptom.

[bob p]

Randall YGPM.

[Malcolm Irving]

... . When you drive an amp through the preamp and run it up to clip, chances are the clip is in the preamp, not the power amp. ....

But surely, if that is the case the amp would not be able to produce its rated clean output?
With all volume and tone controls at max. and the correct magnitude sine wave into the input jack, you should be able to get a clean sine wave at the output at rated power level. Increasing the amplitude of the input sine wave should then cause the output stage to clip first (IMHO). If a preamp stage or the PI clips first that seems to be a bad design (for a clean amp).

[Enzo]

I don't find that to be the case.

[bob p]

Me neither.

Fender test conditions place most of the panel controls on 5. Just turning up the treble control will result in marked distortion of the signal.

[J M Fahey]

It is putting out 39 clean watts with the 4 ohm OT into an 8 ohm load.

Then that amp is working fine, it will put out somewhat more with the proper load, in any case cook a 4 ohm load and remeasure to be certain and *then* order the new OT.

Go to the old Mom and Pop hardware store and buy some raw heater resistors for a couple dollars, measure and cut them to size to get 2 x 8 ohm ones, use screws and terminals as contacts because Nichrome can not be soldered and slide a tile under them because they will get hot and you donīt want to burn anything.

Asbestos would be best but does not have good press lately.



or recycle some from a broken quartz tube infrared heater



or get some intended for electric ovens:



if you also get the ceramic slabs, even better.

Just cut/tap them to get 4/8/16 values.

And they can dissipate hundreds of watts, by design , just be careful where do you sit that hot slab on.

[bob p]

There were times when I was testing high power SS amps and I was so desperate that I had to use a toaster as a high power load box. I've heard stories of people scavenging the nichrome elements out of old clothes dryers, space heaters, etc.

1800W space heater at 125VAC --> about 8R.

[Dave H]

Fender test conditions place most of the panel controls on 5. Just turning up the treble control will result in marked distortion of the signal.

Which circuit did you have in mind? On the SR Normal channel for example the first stage to clip after the treble control will be the power amp.

[cjenrick]

late model cut sheet>

[cjenrick]

same chart only different>



Tweed Champ: 4 Ohm Triad 106

Tweed DeLuxe: 8 Ohm Triad 108