OT impedance and break up relationship

[Randall]

I have a 66 Super Reverb in because customer says it sounds good, but it breaks up too soon for him. I notice it has a 68 Bassman OT in it, 4 ohms. No other mods that am aware of. Instinctively, I know the wrong OT is going to change the dynamics of the amp, but can't intelligently explain why. Is it just the wrong load on the secondary, or is the primary also part of the equation here?

OK, just cross referenced at Classic Tone (Mag Components) and both of their re-pros for Bassman and SR show the same 4.2K primary. So, that's not it.

[The Dude]

I would first measure how much clean power it puts out. Maybe the guy is just trying to get more than it can deliver cleanly.

[Chuck H]

It's worth noting that the 4.2k primary isn't accurate relative to the secondary rating. A 4.2k primary intended for a 4 ohm load becomes a 2.1k primary when applied to a 2 ohm load. This would seem to be the case with the OT you have. The lower load will work the tubes harder and reduce output efficiency. It may also overdisssipate the screen grids because with a low plate impedance the screen circuit impedance can more easily present a lower load for dissipated watts. That can also reduce efficiency prior to distortions. The only ways to idealize (and achieve maximum power) would be to change the speaker load of the SR to 4 ohms (4x16 ohm speakers) or replace the OT with one designed for a 2 ohm secondary.

[bob p]

At the risk of stating the obvious, when you ask about early distortion in a Super Reverb you need to tell us what kind of speakers are in the amp.

The original speakers for that amp were the Jensen C10R, Oxford 10L6 and CTS 10. They're all dirty sounding speakers that break up early. There's no way they'll give you loud and clean.

When I think of a BF Super Reverb, loud and clean is not something that comes to mind. That amp is famous for giving it up early because it's got a quad of low powered speakers that break up early. It doesn't take many watts before that amp gets to snarling with a lot of speaker distortion coloring the sound.

IMO the main reason that people love the SR is because it starts singing long before it gets all that loud. I've never thought of SR as clean amps, and speaker selection is a critical consideration when it comes to shaping their sound.

In addition to checking the amp for clean output and paying attention to the load matching on the OT, I'd try driving the amp into some other cabinets to get a better handle on where the distortion is coming from. My SR is a completely different amp if I drive it into a quad of EVM12L instead of it's CTS 10. With the CTS 10 it's a snarling dog, but with 4x EVM12L it sounds like a Twin Reverb.

[Randall]

The customer is an accomplished blues player with a stable of vintage Fender amps, so I take him at his word when he says it breaks up too quickly. I don't think he is looking for a loud and clean amp like a twin. His band is not real loud. The speakers are 66 CTS, just like what I had in my SR, so I know them. I don't have a 2 ohm dummy load, so I'm not sure how to measure output. And I don't guess putting my 8 ohm load on a 4 ohm OT will give meaningful results. I think convincing him to replace the OT is what I will pursue. Start from there at least.

[Leo_Gnardo]

I think convincing him to replace the OT is what I will pursue.

Proper OT gets my vote as the best plan. A couple other things to look out for: 1. Make sure all 4 speakers are working. and B) I often find ultrasonic hash accompanying low notes. Makes for dirty sounding audio. Even with a 2 ohm OT installed, check for this by sweeping low frequencies while you observe the output on a scope AND while driving speakers not a resistive load. Some SR's have a 47 pF cap across the drive tube's plates - you can increase this up to 470 if necessary to quench the ultrasonic noise.

[The Dude]

Also, make sure all speakers are in phase and nobody's miswired the cabinet.

[bob p]

Based on Randall's most recent post it sounds like the speakers aren't the problem and the problem is in the amp. At least we know where to look for the problem now.

I don't have a 2 ohm dummy load, so I'm not sure how to measure output. And I don't guess putting my 8 ohm load on a 4 ohm OT will give meaningful results. I think convincing him to replace the OT is what I will pursue. Start from there at least.

I'm going to make a few suggestions that might help over the long haul. If they sound critical, please don't take offense at them because I don't intend to be offensive.

If you're going to hang a shingle and do repairs for stores, then you need to fully outfit your bench. That means that you're going to have to buy a few things that it sounds like you may not already have:

1. DUMMY LOADS

IMO every properly equipped test bench needs to be able to test any UUT at full rated power output with working dummy loads. That means that you need to go out and buy all the parts that you need to build a load box that will handle all of the amps that you need to test. Vitreous enamel resistors aren't at all expensive. If you shop around on ebay it'll be no problem to find the parts to build a switchable load box that can handle 2, 4 and 8R loads. Personally, I built a load box that's capable of handling 1kW sine wave drive at 2/4/8/16 because I test some big ass SS amps. If you're primarily going to be working on Fenders and Marshalls then 100W ohmite VE resistors will work fine. Buy two 4-ohm resistors and switch them for parallel, solo and series to get 2/4/8R loads. They're so cheap that there's no excuse for not having a proper load box.

2. SPARE TRANSFORMERS

I'd also recommend buying a generic 50W Bassman OT and keeping it around for testing purposes. Get one that has a 4k2 primary and 2/4/8 secondaries for Fender work, and maybe one that has 4/8/16 secondaries for Marshall work. Having a spare OT like that around is a real time saver. These things can easily be had for $55 or so and are a great investment for troubleshooting.

Once you've got a proper substitute transformer and a proper load box, you'll be able to avoid making embarrassing statements like "I don't have the equipment it takes to test the amp under power" or "let's try replacing the OT because I don't really know if that's the source of the problem."


If you had that extra Bassman OT sitting around you could test the amp with the proper load and get the right answer without having to suggest to the customer that he spend a lot of money in an unnecessary parts hanging exercise. People don't like spending money blindly hanging parts if that doesn't solve the problem. By investing a little money in a spare OT you can do the test work yourself. You can also avoid having that conversation where you recommend a parts hanging exercise because you've already done it. And better yet, you can avoid having people tell their friends that you didn't know how to fix their problem. That would be bad for business.


On to troubleshooting: If an amp is producing distorted sounds and the source isn't immediately obvious, then my favorite brain-dead solution amounts to divide and conquer -- trace signal through the amp stage by stage to determine where the problem is coming from. That always points to the stage that's causing the problem.

[Jazz P Bass]

You really must define "too early".

I have seen these amps start to breakup at '3-4' with hot pickups.

Forget the OT for now.
Get it on the bench & scope it with a 100mv input signal. 1K
4 ohm load.

Look at the output voltage when it starts to clip.
Measure it and report back.

[Chuck H]

Agree with Dude and Jazz. It would help to know just how much of a discrepancy you're actually dealing with.

How much does a '66 SR put out anyway? About 45-47W I'll guess.?. Could be 50W. The circuit looks like it should support that figure, but they so often don't.

[Randall]

OK. Lumps taken and accepted for only having an 8 ohm 100 watt dummy load. I understand it should be better, but that's what it is at the moment. So Bob P and Jazz, thanks, but I am unable to do what you suggest. I posted my question in the Theory section because that's what I was wishing to know. I'm going to put the proper 2 ohm OT in this amp before I try to solve anything else, including measuring output power. It's a perfectly clean 66 AB763 SR, so IMHO, it needs the proper OT. Thanks for the help, I will report back once this is resolved.

[Randall]

In my experience it's closer to 35 watts. On a good day.

[Chuck H]

Well you wouldn't want to take this headroom thing too far. Since your guy is a blues player he may have enjoyed some early breakup, but now there's too much.?. In other words, did he ever like the amp in it's current incarnation? It might be possible to OVER fix it and leave the guy with a cleaner amp than he wanted.

[bob p]

How much does a '66 SR put out anyway? About 45-47W I'll guess.?. Could be 50W. The circuit looks like it should support that figure, but they so often don't.

I have old notes that provide a quantitative answer to that question.

For a 1975 Silverface SR with a 2/4/8 OT on the 8R tap: 20.1V at 5% THD into a 10R resistive load.

That's with a clean sine wave input under the standard test conditions suggested by Jazz. Distortion measurements were taken on an HP 334A. Those results are essentially no different than any of other 40W Fender amps that use the 40W PT.

As has already been mentioned, overdriving the input confounds the issue if you don't have a distortion analyzer. And we all know that the biggest difference between the Fenders is caused by their speaker complement. The stock SR speakers do not help it keep clean tone.

[Enzo]

Did you have him demonstrate the problem? Just what does breakup mean? You instantly focused on impedance at the speaker, but he could just as easily have a problem with a stage clipping early or maybe the phase inverter is not working on one side so when it goes into AB, part of the waveform is missing. A not unusual problem in the Hot Rod series - open plate resistor.