PA060 Hammond variant - did I get stiffed?

Hi all!

So..  I'm overseas in Italy where transformers for ST-70's are hard to find.  Ran across one that seemed to fit the bill.  I was quite pleased when it arrived, but after looking closer the goods delivered were not what was advertised on their webpage https://www.laboratoriomusicale.net/it/dynaco-pa-521-per-st-70-trasformatore-di-alimentazione-versione-originale

After chatting with the owner, who basically shoved me off saying I got what I ordered and that the transformer was perfect, that Hammond specs are conservative etc etc, I wanted to check in and see what limitations I might have using this transformer, along with what modifications I will need to take into account when building.   I have both of Bob's driver boards, 12AU7 and 6SN7 I purchased from him last time I was in the US along with attenuator, and a couple other goodies.   I would like to be able to try both driver boards.  Will use an 5AR4 rectifier and EL34's.  Also have Pavel's auto-bias board.

Regarding no center taps on the filament windings, I saw that a 100 ohm resistor on each leg of the filament windings going to ground were suggested elsewhere on this forum to reduce of hum. Don't really have an issue with that, but am more concerned about the 3.5A ratings for the filaments, and 3A for the rectifier.  Secondary is also higher at 760VCT instead of 720, but didn't test it yet. The US PA060 replacements advertise with 5A for each filament winding and 4A for the rectifier.  Has a 50mm / approx 2 inch stack.

I have two options, either keep and use the transformer or fight for a refund.  Thoughts and suggestions what to watch out for during the build are greatly appreciated!  Anyone else here have the Hammond H302135?  I guess it's a custom order.

If it fits physically then i would use it.  It's always a negative action to try to reclaim, it will take time
and while it's processing you get no new transformer.

BTW, there is an italian guy that actually makes PA060 replacements, i have not the details
at hand. I think he had a site name containing "dynaco" something.

Please send a note here how it goes.

peterh,

It does fit well.  I am building two ST-70's and already have a transformer for the other so no rush at the moment.  I purchased this Hammond, that should have had the same basic specifications as the other at a modest price premium because of the Hammond branding and flexibility with the primary in case I relocate back to the US sometime in the future.  Had specs been delivered as advertised, I would have considered even purchasing another and sold the big one I have so I could have the same transformer in both ST-70's.

Aside from obvious disappointment with the vendor for very misleading advertising, bottom line is that I don't want to corner myself and loose future flexibility as far as driver board and tube combinations go. I wanted to be able to do some side by comparisons over time to find the tube/driver combinations I like best.  With good intentions, it seems instead I ended up in a rabbit hole.  

I just don't feel comfortable at the moment, despite assurances of a vendor I lost trust in and gives no guarantee if I try, with driving the filament windings 50% or more beyond their rating which seems quite possible with some tube combinations.  Thus, my post here seeking some experience before I commit and starting snipping leads or send it back. Maybe I'm overthinking it? Kinda reminds me of a song by The Clash.. "Should I Stay or Should I Go"      

Eventually, the ST-70's will end up with my two sons and I don't want one of them to feel short-changed with an inch of 'stack' less than the other. Hmm..  Does size really matter?     (Hammond on the left)

Anyhoo... kind thanks for your help and suggestions with this little challenge.

Of course the lamination cross-section matters. The smaller transformer will make more distortion and handle less power, all else being equal.

peterh wrote:If it fits physically then i would use it.  It's always a negative action to try to reclaim,

I respectfully disagree. Any seller who is misleading should be held to account.

Brian Beck wrote:Of course the lamination cross-section matters. The smaller transformer will make more distortion and handle less power, all else being equal.

peterh wrote:If it fits physically then i would use it.  It's always a negative action to try to reclaim,

I respectfully disagree. Any seller who is misleading should be held to account.

It's a mains transformer that is in focus, thus distortion is no issue.

In that case, core temperature could become too hot and incipient core saturation can emit hum fields. I still say hold these suppliers to account.

Both the low voltage windings (5.2 vac, 6.6 vac) of this transformer are high, not sure what the normal "wall outlet" voltage is in Italy, but the transformer appears to have a 100, 110 and 120 volt input windings, these may be used to adjust the 6.3 and 5.0 voltages to the correct range. You want to keep these accurate as they affect tube life. Remember the ST-70 was originally designed to run at 117v so it runs with a higher than normal voltage today.

The other concern is the high voltage windings. First it has a 300 ma output (actually very good), not really a problem but it means the voltage drop will be less than what was expected from the Dynaco PA-060 transformer. Add this to the extra 40 volts of AC means 50-70 volts higher on the DC supply, no problem for the B+ output to the tubes, but definitely a problem for the driver board. You will need to increase the resistor between the 2nd and 3rd sections of the cap (believe it is a 6.8k, 2 watt) to get the right voltages on the 7199. I would also replace the cap with either a DynaKit Parts 80/40/30/20 uf @ 550/600 volt unit or better still, the updated version of the classic SDS Labs, Dynaco ST-70 Capacitor board with 40-195-135-135 uf from Triode Electronics. Both have much higher voltage ratings.

The higher voltage on the output tube will give you slightly more power, you will probably want to decrease the bias voltage from 1.56 v to perhaps 1.40 v or so which will also improve tube life.

Three other "suggestions": add a individual tube bias kit from Dynakit Parts, add an Amphenol CL-80 NTC to the power line to slow the inrush current on turn on ( or better still, (2) CL-90s in parallel ) and make the cathode measurement resistor in the Bias circuit a 1 ohm 1%, which when using a voltmeter on the 200 mv scale would read the tube current in milliamps. The 1 ohm resistor decreases the internal impedance and tightens up the sound.

YMMV... Enjoy it

the other problem I see is the limited filament current, only 3.5A each winding. If you use EL34s they use 1.5A each, so that's 3A, leaving only .5A for each half of the driver board.
the 12AU7s on the VTA board use 150ma each, so no problem there. But not advised to use the 6SN7 board, as each one needs 300ma, so at least one of those two windings will be drawing
at least 3.6A of the 3.5A rating. As said in the post above, you'll need a larger dropping resistor on the quad cap (or other cap whatever you are using) to drop the ~460vdc down to around 360-380vdc
for the driver board. The driver board uses 25ma, so simple math (ohms law) suggests a 3.3K 3w resistor will drop 82v.

Hi Tubes4hifi,

Don't know what driver board he'll be using... Understand your concern about the current draw on 6.3 v windings, I was thinking (always dangerous) that a .1 amp overload would be no problem given that the High Voltage secondary windings were designed for 300 ma. The ST-70 is not going to use anywhere near 300 ma, figured that .1 amp could be made up from the lessor High Voltage draw without saturating the transformer core. Additionally the 5AR4 is only drawing 1.9 amps of heater current of the 3 amps available, so that will also allow a slight additional load on the 6.3 v taps. Looking at the Hammond transformer stack, it appears substantially larger than a traditional Dynaco stack as well.

But maybe I'm wrong...

Although the lamination stack height seems Ok (is same as current reissue from Dynakit Parts in the US which I'm using) the Hammond model is suspicious.  Firstly, the photo in the ad from the vender that you purchased it from is of a Hammond 290EX - a different spec'd transformer than PA060; Secondly, there is no record with Hammond or anywhere that I could find on the web of Hammond producing an H302135 transformer or one with those specs; Thirdly, the 5.2 Volt and 6.6 Volt windings versus the typical 5.0 and 6.3 are odd.  I'm wondering if this unit is a counterfeit of questionable specs and quality.  I suggest that you power it up and measure voltages both open circuit and at rated load.
Jack

SIZE DOES MATTER! It appears that the Hammond transformer in question is half the size of the original. More laminations means more current (power) before saturation. My fear is that the Hammond will overheat and fail early.

Hammond also makes custom transformers. It is possible that this was surplus from a custom run of transformers.