How Close Can You Safely Get to a PT's Current Rating?

[Alexo]

Good morning everyone,

I have read that Hammonds are conservatively rated, but I am sitting next to an SE, triode connected EL34 stereo running a 200 ma Hammond PT at about 100ma and by the end of the day that pt is pretty much too hot to touch for more than a second.

I also built a little amp running 2 6AQ5's at 180 volts for 64 ma on a Hammond PT rated for 65 ma (suicidal, I know) and that thing holds up but it too gets really freakin' hot.

So the project I am working on now is going to draw about 230ma idle current from a 250ma PT, is this asking for trouble? Does it mitigate the situation at all if I don't use the heater taps on this pt?

I know it would be ideal to use a 400ma pt but I couldn't find one with the right voltages...

Thanks!

[David Root]

Hammond's DC mA ratings are considered to be conservative, according to what I have read. I wouldn't worry too much about temperature.

I assume you are using the Hammond 200 series, which are designed for 117V AC. Just about everywhere these days wall AC will run 120 to 125 VAC, so you're slightly "overpowering" any Hammond 200 series PT.

Pulling 230 mA from a rated 250 mA PT is quite safe. Not using the heater tap might very likely reduce the operating temperature, as this is designed to draw serious power, eg 4A*6.3V=25.2 VA, on the order of 25-50% of the HT winding VA.

You may find the Hammond 300 series more to your liking as far as temperature goes. They are heavier, but they have international AC primary windings you don't need, BUT they are designed for 120 VAC operation, a good thing.

You might also try PTs which are specifically designed for musical instrument amp applications. Hammonds are not. By which I mean something designed to accomodate the large current draw variations in the class AB1 operation most MI amps operate in, especially when being leaned on by a hot humbucker and a dimed amp. Having said that, one of the easiest ways to start an MI amp argument is to suggest that Hammond iron is, or is not, suitable for MI amps!

On the 400 mA PT, not necessarily. If you put a Fender Bassman PT in an old Vibrolux, it won't necessarily SOUND better, and in fact could sound worse than the correct transformer. (OTOH, I do like to oversize the OT a bit which gives better bass response. This is subjective of course and depends on how the rest of the amp is configured).

[Alexo]

Thanks David, that's good to hear, especially since I just wired up the pt last night.

It is actually a 363CX and I am deliberately overpowering it by a hair, running 125 into the 110 tap to get just the right secondary voltage, does that mean the current limit will decrease proportionally to the voltage increase?

This thing doesn't have a 6.3 tap, only 5 volts at 3 amps, so I got to make a further donation to Hammond and get a filamtent transformer.

Can't hold it in any longer here's my project: a 6x6AQ5 amp for thirty watts, with a rotary switch on the front that grounds out the grids of different power tubes so it can be set to 5, 10, 20 or 30 watts. I guess if the pt gets too hot, I could just yank out a pair of power tubes, but the unused sockets would really bug me.

[David Root]

I had a feeling this was not as straightforward as it seemed! OK, 125VAC into a 110V primary winding is a bit more than a "hair"!

As to its effect on mA DC rating of the HT winding, that really depends on the current capacity of the wire in that winding, i.e. its diameter. Now the Hammond 300 series looks like a little heavier duty design than the 200 series, but it probably is not designed to absorb that much of a primary overvoltage without some derating of the HT winding's DC mA capability. Remember, the original VA rating of this winding hasn't changed, so the answer to your question comes down to how conservative is that VA rating, and that depends on the wire diameter "headroom" over the minimum required. Put in terms of Service Factor, if that wire diameter has a 1.15SF on rated VA, you're going to be OK. If it's 1.05, you're pushing it a bit. There are other design factors that could possibly affect this situation, but I'm not knowledgeable enough at that level to go there with any confidence.

I'll come back to you shortly on your unique circuit, have to do a couple things right away, then take a good look at it.

[oldhousescott]

Hi Alexo,

I would be tempted just to hardwire the grid connection on the upper left output tube since it is on in all positions anyway. That would be just one less switch contact to potentially go bad over time.

Hope this helps......

[Alexo]

Well I just got off the phone with Hammond -

They will not release the SF number to anyone, but I did have an interesting conversation. The rep said going 10% over primary voltage was about the safest you could do as a rule of thumb but that you really have to carefully monitor the temperature of the ambient air and the core if you go beyond that, not exactly something I am equipped to do. He said I didn't really have to de-rate the current rating but that the VA will obviously rise.

He also told me that they were re-designing the 200 series, which are still being made from the original 1940's designs (!) they will soon have 120 or 125 volt primaries, will be interleaved and probably have better cooling.

[Alexo]

Hi Alexo,

I would be tempted just to hardwire the grid connection on the upper left output tube since it is on in all positions anyway. That would be just one less switch contact to potentially go bad over time.

Hope this helps......

Great recommendation, thanks.

[David Root]

My ISP went down while I was composing my notes on your circuit. Just got it back.

Interesting circuit, to say the least! Given the low B+, cathode biased 6AQ5s, this no doubt sounds pretty "old school".

How did you arrive at the 3k3 OT primary? It seems reasonable given all the options. Some time ago I built a tweed Harvard using all 7-pin tubes, including 6AQ5s. It roars. Curious about the 5k6 gridstoppers, I used 1k5.

470K/0.047 on grid resistors/coupling caps. Same time constant as old Fenders, but I'm guessing you used 470k grids to boost the gain a bit for six tubes.

PI with channel 2 coming in on the opposite grid from channel 1, why not, there's no feedback to plug in. 470K grids and 47K tail for good balance.

Tweedy looking tone/volume. Channel 1 first stage using what amounts to a tweed mixer, and EF86 pentode channel 2 with more coupling cap choices than an old Boogie. I also see lots of pio caps, which I happen to like too.

What speaker(s) are you using with it?

Thanx for the notes from Hammond, that was interesting, and I wasn't too far off on the SF estimate either. I had been wondering when they would update the 200 line.

[Alexo]

Thanks a lot for the comments, David!

Interesting circuit, to say the least! Given the low B+, cathode biased 6AQ5s, this no doubt sounds pretty "old school".

That's the way I like it, plans are in the works for a Dumbley amp but I kind of need this thing done for an upcoming gig (my wedding in 2 weeks) so I decided to stick with something simple and time-tested. The B+ is low but from what info I can find, 6AQ5's really don't want any more than 275 max on their plates, the PT set up with the 110 volt tap put out about 281 unloaded, so I guess using the 120 tap will give me around 250, although I am hesitant that it will drop under load...

How did you arrive at the 3k3 OT primary? It seems reasonable given all the options. Some time ago I built a tweed Harvard using all 7-pin tubes, including 6AQ5s. It roars. Curious about the 5k6 gridstoppers, I used 1k5.

Well a lot of this has come straight from the datasheets: 10K load at 250 volts, so 3 pairs would want a 3K3 load, I'm using the higher grid stoppers because the tubes don't take much to overload and I am hoping they will be a little less prone to blocking distortion this way. I really like 6AQ's too, I'm building little single-ended amps with them to give to my friends in return for their services as a pick-up band for all the other friends of ours who've agreed to come up and play a couple tunes at our wedding-palooza. Bet that Harvard's a rocker...

470K/0.047 on grid resistors/coupling caps. Same time constant as old Fenders, but I'm guessing you used 470k grids to boost the gain a bit for six tubes

Yep and yep! This guy may need to double as a bass head from tme to time so there you have it, the power section looks pretty funky for bass, but my 5E3 head actually sounds pretty good for bass with all that compression, so maybe this one will be Ok.

PI with channel 2 coming in on the opposite grid from channel 1, why not, there's no feedback to plug in. 470K grids and 47K tail for good balance.

Tweedy looking tone/volume. Channel 1 first stage using what amounts to a tweed mixer, and EF86 pentode channel 2 with more coupling cap choices than an old Boogie. I also see lots of pio caps, which I happen to like too.

As much as I would like to take credit for all that, I owe about 90% of it to the great builders at 18watt.com, this is basically a combination of a couple different versions of that amp with my hopefully neighbor-friendly adjustable output section, and of course the 6AQ5's which I like more than EL84's. PIO's are my favorites, I would have used them for everything but I limited myself to the values I could get for around $5 apiece from Angela Instruments. Little known that you can get them in 400 or 470 pf and use them in tone caps so you can avoid silver mica.

What speaker(s) are you using with it?

A pair of Sammi's in a Reason 2x12 cab, something else I picked up from 18watt.com!

Thanks again for the help, I'll try and post a clip when this thing gets up and running, now to rewire that primary tap..

[philmanatee]

That's great, I'm actually working on a six 6aq5 amp myself right now! I really fell in love with the 6aq5's after I got a little selmer combo that used them. I can't wait to hear how your amp sounds. Phil

[Alexo]

That's great, I'm actually working on a six 6aq5 amp myself right now!

No kiddin???



Hey, what kind of power transformer are you using?

[philmanatee]

I'm using an onetics ac-30 power transformer, and his 30watt high def output. I was surprised to see someone else that likes 6aq5's, let alone doing a six banger too! I'm still drilling my chassis and making changes, but I hope to be bolting parts in and soldering this weekend. Phil

[klingo]

hi!
i'm very curious about the headroom a 6AQ5's "18watt" could do, as my el84 18lite in a blues jr cab is way to loud for rehearsal (still a bit loud with an Hot Plate at -8db...and crappy sounding at -12 db).
We are playing ACDC with a "not very powerfull" drummer and i'm playing the malcolm stuff.
I already have a 220v/6.3v PT, choke,6005 & sockets, and the blues jr OT (or a MIG50 OT 4k8/4/8/16ohms)....
So do you think it could do the job... good rock tone with less attenuator
may be less(10watt) is more(than 18watt)

I notice that there is no caps between the PI grids and Volume pots in your schematic...so i'm still searching which value will be good here...22nf?

also curious about the capacitor selector switching noise (matchless use 4.7/5.1 megaohms resistors but put selector after caps)

tell us about the 6AQMonster5 sound...

cheers

[Alexo]

So do you think it could do the job... good rock tone with less attenuator
may be less(10watt) is more(than 18watt)

I notice that there is no caps between the PI grids and Volume pots in your schematic...so i'm still searching which value will be good here...22nf?

also curious about the capacitor selector switching noise (matchless use 4.7/5.1 megaohms resistors but put selector after caps)

Well I built a 5 watt 18 watt running 6AQ5's at 180 volts for half power, it is just barely loud enough to keep up with a band when running dimed through a 4x12 cab, I think 2x6AQ5s at full 250 volts for 10 watts would be just right.

Good catch on me missing those caps to the pi!!! I always forget about those for some reason, I think I'll go with .047 to keep with the lows in this build but I may end up swapping them around and experimenting, maybe leave one channel .047 and the other .022.

RE: the switching noise, AES has make-before-break switches, so it shouldn't be a problem. I know 12 pole is kinda silly but what the heck, I've got the caps...

philmanatee - I really thought I was the only one too, what are the odds? Good luck with your build!

[klingo]

thanks!
metalwork in the offing...