Is this old 60's valve radio PT suitable for PP 6V6 build?

[dwrockdoctor]

Hi all,

I scored a few old 60's aussie valve radio PT's off ebay for a steal and am wondering if they'll suffice for a PP 6V6 amp build using JJ 6V6s and a few 12at7 preamp tubes (build based off of an old Orange OR120 circuit modified for low wattage).

From an old article I found on how to deduct the milliamp rating of the high voltage secondaries, I came up with 90ma for one and 75ma for the other. From markings on all the connections it seems that high voltage secondaries are 385-0-385, 6.3V heaters at 3A and 5V rectifier at 2A. A bit confused on the ones marked E and C, I'm guessing Earth and Common and how do I best use these ones?

I was going to have the JJ 6V6s push/pull connected for both pentode and triode at the flick of a switch, probably will use a SS rectification also.

I know that the JJ 6V6s can take high plate voltage but will the 385-0-385 secondaries be pushing too much voltage onto the plates? I guess I could always use a zener set up to reduce voltage. I'll most likely use a Hammond 125E as the OT.

Anyway, here's a photo of one of the transformers with voltage values, let me know if this will be okay for what I plan to build, I have built three tube amps to date so far.

[Chuck H]

The 5V winding is evidence that whatever tube was intended, there was a rectifier tube (and it's associated voltage drop) involved. It can depend on what primary voltage you'll have relative to what primary voltage was originally intended, but running a quick and rough simulation on Duncan's PSUDII (averaging a guess for plate current relative to the high plate voltage) I get a calculation of 430 to 440Vp. A bit high, but not out of the real world for amps like vintage Fenders with a 115V primary seeing 125V. Which happens all the time here in the US. (these amps used a 350-0-350 PT with a 115VAC primary that is now seeing 120 to 127VAC) Modern 6V6's that are capable of these higher voltages have thus far been the solution for owners of these amps. If these PT's you have are rated for a lower primary input than your wall puts out then you have an additional complication. Some voltage reduction (like Zener diodes) is probably in order. Just as important in the case of higher wall voltages would be to mitigate excessive filament voltage as this can greatly reduce the life of the tubes.

[dwrockdoctor]

Okay great, thank you!

Hooking up the transformer unloaded and taking some multimeter readings may be in order then.

Also, is 90ma on the secondaries (and the 2nd transformer of 75ma for that matter) enough current to run two JJ 6V6s and two 12AT7's plus screens?

[Chuck H]

Okay great, thank you!

Hooking up the transformer unloaded and taking some multimeter readings may be in order then.

Also, is 90ma on the secondaries (and the 2nd transformer of 75ma for that matter) enough current to run two JJ 6V6s and two 12AT7's plus screens?

The 90mA unit will be ok. Many OEM guitar amp PT's intended for a pair of small bottles are rated there. The 75mA unit might do well to skip using the 5V winding to reduce heat in the overall unit. Dropping voltage (if necessary) with Zeners instead. They'll probably be ok then.?.

Also, you might be able to reduce voltage some just by choosing the higher close primary. That is, If you have 230V from the mains choose the 240V tap. If you have 250V from the mains choose the 260V tap. And, again, do test the filament voltages. They should read a little high unloaded. If they read at rated voltage you should be ok too. A margin of more than 10V difference between your mains voltage and the primary tap chosen could cause filament voltages to be off by more than 10%.

[Mick Bailey]

How did you arrive at the current rating? Radios generally run plenty of small tubes - especially if it's a superhet - but only have a moderate current requirement. Usually the output is SE and even just a few hundred milliwatts was enough output to be 'too loud'. That said, there are a few high output P-P radios. Have to say that your picture looks more like an 'equipment' transformer than something specifically for radio. I have a few Muirhead ones the look very similar.

C looks like the common connection - sometimes marked as 0v. Also, often there's a screen between primary and secondary that's connected to ground - that could be your E terminal. Check to see if it's also wired to the frame of the transformer and (to be sure) to no other terminal.

JJs are tough tubes and are the 6V6 equivalent to the Sovtek 5881 in terms of voltage handling and reliability. If you make sure the current draw is OK there shouldn't be any need to reduce the voltage. Bear in mind that your loaded voltage will drop, depending on the resistance of the HT winding.

[g1]

The C appears to be the low side of the primary, for which you have a choice of 220, 240, or 260VAC.

[dwrockdoctor]

I found an old article about figuring out values of unknown transformers. I divide the resistance of the high voltage secondary with the no load AC voltage of the secondary and then using the resulting answer with the graph (which I've attached to this reply) I'll arrive at the milliamp rating. I put my own curves on the graph based off of known milliamp ratings of other transformers I have.

[Mick Bailey]

You can cross-check by using the method in post #15 here; http://music-electronics-forum.com/t44833/

[bob p]

The hardest part about doing that is that you need a very accurate way to measure a low DC resistance. Because the DC resistance is a small number that ends up being used in the denominator of your calculations, errors in the measurement can introduce significant error in the final result.

Unfortunately, most battery powered meters aren't all that great at measuring low DC resistances with high precision because they have to generate significant current to take the measurement. IME an AC benchtop meter works better, and a 4-wire kelvin meter works best, if you can get your hands on one.

Once you have that DCR number, PSUD is very useful in modeling the transformer's performance.

[Chuck H]

The hardest part about doing that is that you need a very accurate way to measure a low DC resistance. Because the DC resistance is a small number that ends up being used in the denominator of your calculations, errors in the measurement can introduce significant error in the final result.

Unfortunately, most battery powered meters aren't all that great at measuring low DC resistances with high precision because they have to generate significant current to take the measurement. IME an AC benchtop meter works better, and a 4-wire kelvin meter works best, if you can get your hands on one.

Once you have that DCR number, PSUD is very useful in modeling the transformer's performance.

The primary can, indeed, measure very low. A decent meter measured for it's default by touching the probes together should give a useful reading (subtract the default, or "null" that reads on your shorted probes). And the reason I'm posting is that along the same lines...

Sometimes a signal generator is called into service for determining the actual impedance and/or ratios for a given OT. You will absolutely want to measure a signal generators voltage with the unit under test. Setting the signal voltage before hooking it up and then NOT re measuring that voltage will result in inaccuracy because the transformer presents a load that will reduce the actual input voltage. The same concept should apply to testing PT's in any similar way. Always measure voltages with the unit in test.

[Mick Bailey]

I'm referring to measuring the secondary DCR, though - often some 10s of ohms and in radio transformers can be comparatively high.

[Chuck H]

Oh, I know. I was just outlining some principals like meter null subtraction and testing voltages in situ. Early on in my learning I was confused at times when I failed to recognize these important details.

[Mick Bailey]

Meter null subtraction grinds my gears because it doesn't have to be this way. I have a friend who many years ago was given a novelty promotional DMM from some cable marketing company - a slim thing the size of the original i-pod with just a single button. The really nice thing was if you held the probes together and pressed and held the button it zeroed the lead reading. You could also do this with the probes across a resistor and it would zero the reading and then give +/- measurements. It auto-ranged, auto selected (no need to press for resistance or voltage) and if you had AC superimposed on DC showed them both. You could also override the auto functions and select manual ranges all off the one button.

[eschertron]

IIRC all analog meters had a pot for nulling resistance readings. Part of the bridge circuit, I assume. When the technology changed and digital came out, that feature wasn't an essential part of the device, and *adding* it would add cost. Yes, the dingus we use is fancier than the ones we had 30 years ago, and there's no reason not to add that particular benefit back in.

[Chuck H]

Meter null subtraction grinds my gears because it doesn't have to be this way. I have a friend who many years ago was given a novelty promotional DMM from some cable marketing company - a slim thing the size of the original i-pod with just a single button. The really nice thing was if you held the probes together and pressed and held the button it zeroed the lead reading. You could also do this with the probes across a resistor and it would zero the reading and then give +/- measurements. It auto-ranged, auto selected (no need to press for resistance or voltage) and if you had AC superimposed on DC showed them both. You could also override the auto functions and select manual ranges all off the one button.

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