Misunderstanding properly matching guitar amp to output transformer to speaker

[Potato Cakes]

Hello, Everyone,

I've recently been enjoying building point to point guitar amps and thought I had a pretty good handle on making sure the output transformer and speaker selection would work well with the amplifier circuit. My third amp build is a Vibro Champ. This is a 5W single ended amp using a 6v6 for the output tube. The transformer I bought was from Mercury Magnetics and listed as a replacement for a Tweed Champ, but this also uses the same schematic and output tube minus the vibrato section, so this transformer should be fine. It also has a single 8 ohm output which is going to an Eminence 10" speaker that is 8 ohms and rated for 75W. After getting the initial build errors corrected, I got this amp fired up and was enjoying playing through it for a little less than an hour when it started intermittently crackling very loudly, to constantly crackling, to no guitar signal and just louder than normal amp hum. I did the whole routine of changing out all of the tubes and rechecking solder connections but nothing seemed to fix the problem. Further testing found that that my B+ voltage would fluctuate greatly and then stop at around 100V (should be about 355V). After more troubleshooting, I found that this problem would persist with no tubes, which I would expect a very high B+ voltage with no load. I disconnected the output transformer primaries and then the B+ voltage behaved as it should. This is the same amp I mentioned in another post that was squealing very loudly because the negative feedback connection was backwards, so I don't know if this damaged the transformer, it was faulty from the manufacturer, or if a mismatch of components caused the failure. I don't believe the higher speaker wattage rating has anything to do with it as I am using this same model with another 5W single ended build and a 15W push pull amp without any issue.

I am measuring 255 ohms across the primaries, which I think is an indicator of a bad transformer since I usually see less than 1 ohm on any other transformer that I've measured this way. On the secondaries I get 0.7 ohms.

In the meantime I've ordered a bigger transformer for higher wattage amps using the same output tube. For the future I would like to know if there is anything else besides wattage rating of the transformer and the impedance of the speaker being used that need to be taken into consideration when ordering components for tube guitar amplifiers.

Thanks!

Paul

 

[emrr]

255 isn't out of line for a single 6V6 transformer. Some are much higher. The B+ swing (measured through the output pri?) would suggest an intermittent (or leaking to ground, have seen this) winding IF you can say with another meter that B+ is solid on the other side. If it's fluctuating everywhere, it could be PSU on it' own, or affected by output problem. I'd want to look at current fluctuation in multiple places (pain to do) also to pinpoint 'where' definitively.

 

[fazer]

Does it use tube rectifier. I would check /replace the tube

 

[Potato Cakes]

The B+ swing is measured at all of the tube plate voltages (with or without tubes installed) and even on the HT windings of the power transformer. When I pull the output transformer and connect the B+ directly to the 6V6 plate and reinstall all of the other tubes the voltage seems to be fine. Although this is only my third amp I've built plenty of studio gear that uses tubes and I've not seen anything like this. My knowledge of testing output transformers is limited so I don't know how to examine it thoroughly.

I'm out of town till next week. The new transformer should be in by the time I get back. I'll know then if I have other problems.

Thanks!

Paul

 

[Potato Cakes]

Does it use tube rectifier. I would check /replace the tube

All of the tubes have been replaced.

 

[Potato Cakes]

It was definitely the output transformer. I swapped it out and everything works perfectly. I took the bad one and measured again and got 300ohms on the primaries and 0.8 on the secondaries. I also got 20M between the primary and secondary windings. It seems to be perfectly fine which is strange that it is the problem. I'll email Mercury Magnetics and see what they say.

Thanks!

Paul

 

[Walter66]

0.8 on the low impedance secondary side may be fine, but on the high impedance primary side it should measure much higher. To expect one Ohm on the primary side couldn't be measured for a working 6V6 output transformer. It depends on the wire size and wattage of the transformer, how low the primary side measures, but usually expect some ohms here.

 

[CJ]

Mercury is probably going to jerk you around ,

Standard vibro champ had a 4 ohm output transformer into a 4 ohm oxford speaker but as long as your using an 8 ohm speaker then you are correct in choosing an 8 ohm output XFMR.

Sounds like you punched a hole in the pri/sec insulation.

Primary (tube side) resistance should be around 250 to 300.

Secondary (speaker side) resistance should be from 0.2 ohms to 0.5. you really need a 4 wire meter to measure this but who cares. The secondary wire is not going to break unless a green comet comes thru the roof and lands on the amp but that only happens every 50,000 years.

In order to buy a output you need the primary impedance which is determined by what power tubes you are using and secondary impedance which is determined by what your speaker load is going to be.

Most aftermarket outputs have multiple taps to allow for different speaker setups.

And like you said, you need the power rating, but most transformer companies spec their output transformers by what kind of amp you are using it for. Or they might spec it by saying "2 ea 6L6 into 8 ohms or something like that.

If you want to use a transformer from another source then you need to spec the wattage or primary current that you will be running.

Tube manuals are a good source for finding impedance specs for tubes, they will usually give plate resistance for a single ended design like the champ and also a spec for plate to plate impedance for two tubes in push pull. If you are using 4 tubes in push pull then you simply divide the number in half.

Now where it gets interesting is the actual ratio that an output will have for a given set of tubes. For instance. If the tube manuals says 4,000 ohms for push pull 6L6 tubes, one amp maker might use a transformer with 5 k ohms pri z and another amp maker might use 3.8 k ohms. Different ratios will give you different sounds as related to distortion and power transfer, this makes experimentation fun as you can go outside the box for weirdness but there might be a price to pay as power tubes might wear quicker if you run a mismatched output XFMR. And tubes are expensive nowadays since Mr potato head invaded Ukraine.

There are a bunch of Fender transformer spec sheets in the magnetics dept of you search for "fender amplifier transformers."

You know I have never taken apart a mercury opt. If you want to pay shipping I will rewind it for you with some real insulation instead of that cheap poly they use. You do not need a more powerful transformer, a 6v6 is only good for 5 watts anyway, if you already ordered a bigger opt then you can use it for something more powerful.

 

[Potato Cakes]

CJ,

I'll send it but you can just keep it. It's actually a ToneClone branded transformer but it is sold as a Mercury product. I won't need the transformer back I don't think I will be building anymore Champ variations. I think I want to focus more on push-pull type designs, which would require the bad transformer to be rewound for something completely different. You can just post any interesting findings for everyone's benefit. If you have a use for it yourself you are more than welcome to it.

The failed output transformer was for an 8 ohm speaker, which I am using. I have built some amps with multiple speaker ohm taps, but the speakers I have been using of late are the 8 ohm, 10" Ragin' Cajun from Eminence. And for this Champ build I didn't want to go crazy with options, but I did add a switch to remove the cathode bypass cap on the second gain stage of V1 to make the amp breakup differently for fun.

I replaced it with a Mojo Tone transformer (Blackface Champ Output Transformer 125A35A). I think Heyboer actually makes them. So far it works perfectly and it was a less expensive than the one I got from Mercury.

I think you also answered a question that I had. When looking at matching the impedance of output tube and transformer in a push pull configuration, the impedance of one of those tubes is what is used for output transformer, correct? And if a transformer with the exact impedance needed can't be found, is it better to use the next nearest impedance that is higher than what is specified for the particular tube being used?

PM me a shipping address and I'll get it send to you this week.

Thanks!

Paul