SRV 8-ohm speaker 2-ohm OT

BoomTexan

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I've been looking around the internet for ideas on making a Brownface Vibroverb with SRV mods. It seems to be common consensus on forums that Cesar Diaz had no idea what he was doing at all, and made some really boneheaded design choices, like setting bias obscenely hot and blowing multiple sets of tubes every show.
However, this one that really scares me in particular: he got a 2-ohm OT (from a bassman or super reverb or similar) and attached an 8-ohm EV speaker. I have read that old Fender transformers can handle a lot of crap, and are built to withstand 1/2-2x the power, but if I were to build one of these amps, would a regular OT from Weber or Hammond be able to handle that mismatch? Was it even safe for him to do it with vintage Fender materials?
 

Axis29

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The problem is, you're incorrect about which Vibroverb Circuit to use.

SRV Owned '64 Vibroverbs. They were blackface circuits.

The bronwface ones are the 1963 Vibroverb circuit. They aren't the same amps and don']t sound the same.


However, I would never do that kind of mismatch without contacting the manufacturer and asking them. Doing a touch of reading, I found this post on Strat-Talk: https://www.strat-talk.com/threads/will-a-vibroverb-64-reissue-w-diaz-mods.320773/post-1818458. He specifically states: "Ok, I am pretty sure if he used a Bassman OT he used the 100W OT and used the speaker tap(not the 8Ohm tap) feeding the 8ohm speaker to match the tubes." It's got a 4 ohm tap....
 

BoomTexan

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No, I absolutely know that he used 64 Vibroverbs, and that those were the backbone of his sound, but I could've sworn that I saw concert footage of him with the brownface one. Looks like I'm gonna have to do a Blackface one now, haha.

Thanks for info about that, you did a lot more digging than I did. I was genuinely concerned that there was a 4x mismatch on there. Diaz's mods are really counterproductive. He raises gain and distortion and then implements other measures to reduce it that just make 0 sense. Still, I'll bet that they have a part to play in getting that tone, so I'll be as faithful to these mods as far as it's not totally dangerous.

I'd be willing to do a 2x mismatch on a good OT, but I could probably get the same result in regards of gain by other methods that are safer. Perhaps I could try to get the tube resistance -> transformer ratio to between the 4-6 ohms region and then slot in the 4 ohm transformer.
 

2L man

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Using too low OT load wear out tubes, where possible high bias current make it even more wearing. Bias current store energy to the OT and the voltage peaks power tube anodes "see" can come so high that there comes spark current inside tube. Sometimes this happen in tube socket if it is dirty.

In normal low distortion operation anode voltage peak to peak exceeds the DC voltage anodes have when amp idles and this Vpp potential increase when bias current is increased.
 

Phrygian77

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There were a few Bassmans and the Vibrasonic in the early '60s that had the big 4k 8 ohm OTs. I doubt that's what he used though. You could use the 100W 2k 4 ohm OT for a correctly reflected load. My Twin Reverb is running an 8 ohm load, initially set up with a just pair of 6L6s.
 

Axis29

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No, I absolutely know that he used 64 Vibroverbs, and that those were the backbone of his sound, but I could've sworn that I saw concert footage of him with the brownface one. Looks like I'm gonna have to do a Blackface one now, haha.

Thanks for info about that, you did a lot more digging than I did. I was genuinely concerned that there was a 4x mismatch on there. Diaz's mods are really counterproductive. He raises gain and distortion and then implements other measures to reduce it that just make 0 sense. Still, I'll bet that they have a part to play in getting that tone, so I'll be as faithful to these mods as far as it's not totally dangerous.

I'd be willing to do a 2x mismatch on a good OT, but I could probably get the same result in regards of gain by other methods that are safer. Perhaps I could try to get the tube resistance -> transformer ratio to between the 4-6 ohms region and then slot in the 4 ohm transformer.

Well, here's a little bit of my perspective on the whole big transformer thing... I am sure some of it was about volume, but I think a lot of it has to do with clarity in the bass frequencies.

I say this because of two of my amps. Same circuit, one's 40-ish watts and one is 80 watts. Same internal components, other than transformers and a couple extra 5881's. The biggest difference between the two amps is how hard the bass hits you. With the big amp, you FEEEEEEEEL it hard in your chest. It's a thrilling, exciting feeling! The smaller amp sounds great, it really does. But, the bigger thump gives you a certain tickle feeling about your own midrange.

I think Diaz went for bigger thump, harder hitting, tone more than anything.
 

Wyatt

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Legend for decades is the SRV BFVV had a TR OT in it, that would pair correctly between two 6L6GC and an 8-ohm speaker. @Phrygian77's theory that it may from a Bassman 100 could be correct, pretty much same transformer as the TR.

A Super Reverb OT will eventually burn out from flyback voltage driving a 8-ohm loud at loud volume. I've had to repair them first-hand. But these days you can get a 2-4-8 multi-tap BM/SR OT and not worry about it.
 

Phrygian77

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@Wyatt yeah, I wouldn't in a new build use the 100 watt OT when there are good Bassman/Super type OTs with multiple secondary options available. An argument can be made that the 100 watt OT has a lower inductance, which could potentially have an effect on the impedance of low frequencies given a higher reflected impedance. There's certainly no noticable loss of low end in my Twin Reverb. Calculating or modeling it should be simple, but good luck trying to find the inductance numbers for that OT at the correct frequencies and voltages.
 

BoomTexan

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Okay, so it seems the consensus is that it could've been a Bassman 100 or another similar large transformer that runs at 8-ohms. My guess is a tech said "oh, yeah, I swapped in a Bassman transformer on Stevie's Vibroverb" and someone took that as a 4x10 Bassman that would run at 2-ohms, told everyone, and the myth perpetuated itself on TGP and other forums.
 

Phrygian77

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I assumed the Bassman 100 and Twin Reverb OT were the exact same part, and they are definitely not the same. The Bassman 100 OT is Fender CBS part number 021657.

Funny thing, I found a 2016 post from @Wally mentioning that Cesar Diaz used a Twin Reverb output transformer.

The small OT in the Vibroverb is a big difference, Ime, compared to the larger iron used in the SR, TR, and the Bassman amps of that time. For instance, when Cesar Diaz overhauled SRv's VV amps, he used a TR OT. the larger iron yields a different sonic and slightly more power than does the smaller OT that Fender used in the Bandmaster, V'verbs, Pro, Pro Reverbs and the Vibrolux Reverbs. Nowadays, the smaller iron might be more desirable because most are not looking for more power.
IMHO, one should consider exactly what they want out of the amp before embarking on such a project because here is a difference in what the different OT's do. Punch and headroom?? LArger iron. Looser and earlier breakup??? Small iron.
Either way, my personal thought is that the impedance should be a match.
 

Tele295

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Twin OT transformer has the taps, right? 100 (80) watts at 4-ohms and 50 watts at 8-ohms
 

2L man

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Twin OT transformer has the taps, right? 100 (80) watts at 4-ohms and 50 watts at 8-ohms
OTs should be able to deliver same power to all speaker loads. Sometimes there is 1 or 2db difference at 80Hz or above 10kHz where all loads begin to drop.
 

Wally

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Twin OT transformer has the taps, right? 100 (80) watts at 4-ohms and 50 watts at 8-ohms

The Vintage TR OT a is a 4 ohm OT. When one runs only two power tubes, the relationship between the primary and the secondary is changed making the OT want to see an 8 ohm load. This has nothing to do with the ability to make power. IF the two power tubes could produce more output, then the OT would accommodate them by yielding more power to the speaker.
 

Phrygian77

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Twin OT transformer has the taps, right? 100 (80) watts at 4-ohms and 50 watts at 8-ohms

The transformer has an impedance ratio (also the inductance ratio), which is the turns ratio squared. The impedance is the load impedance on the secondary reflected to the primary. So, the impedance numbers usually given with an output transformer don't really mean anything other than they are the load and the reflected load impedances that the transformer was designed for. Ideally, the transformer doesn't affect the impedance. In reality, the transformer windings have inductance, which influence the impedance, particularly at lower frequencies.
 

rdjones

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Twin OT transformer has the taps, right? 100 (80) watts at 4-ohms and 50 watts at 8-ohms
The only tap on any Fender output transformer prior to about 1976 is the center tap on the primary.
They all used a single secondary winding without a tap for multiple load impedances.
 

schmee

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No, I absolutely know that he used 64 Vibroverbs, and that those were the backbone of his sound, but I could've sworn that I saw concert footage of him with the brownface one. Looks like I'm gonna have to do a Blackface one now, haha.

Thanks for info about that, you did a lot more digging than I did. I was genuinely concerned that there was a 4x mismatch on there. Diaz's mods are really counterproductive. He raises gain and distortion and then implements other measures to reduce it that just make 0 sense. Still, I'll bet that they have a part to play in getting that tone, so I'll be as faithful to these mods as far as it's not totally dangerous.

I'd be willing to do a 2x mismatch on a good OT, but I could probably get the same result in regards of gain by other methods that are safer. Perhaps I could try to get the tube resistance -> transformer ratio to between the 4-6 ohms region and then slot in the 4 ohm transformer.
I've had no issue with running at 1/2 match. I actually preferred to run a Bassman head (in combo cab) with a big 15" 8 ohm JBL.
My gut says the larger the tranny, (at a given wattage) the better it handles mismatch. I have found in small amps, a mismatch makes for uglier sound and in large amps Bassman>Showman, it seems to matter very little. I don't know if there is any science behind that or not.
 




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