Power Transformer Ordering Questions

[silverfox]

I've gravitated towards the 40 watt output power region for builds and have stocked parts for several builds. I'm now wanting to add a transformer set to my parts.

Unfortunately I can't seem to find a replacement power transformer similar to what I have. In browsing the sites nothing seems to fit and the reason for that is the filament current at 5 amps.

When determining the load capabilities of the filament tap I placed a rheostat across the filament output, slowly increased the load and monitored the output voltage until it began to drop below 6.3vac. The result was about 5 amps.

That has worked well for me as I loaded the test chassis with 5 12AX7's and 2 EL34's, checked the filament voltage and all was well.

Here are the all the specs: 350-350 and about 90 to 120 ma; 50 to 60 vac bias tap; 6.3vac @ 5 amps.

I'm thinking I'd like to add another preamp tube for a total of 6 in addition to the EL34's.

I like the idea of the separate bias windings but may be convinced otherwise.

Any suggestions?

Silverfox.

[DummyLoad]

Hammond 372dx should work.

-pete

[labb]

Can you run 5 12ax7's and 2 EL34's on 120 ma. ?

[SILVERGUN]

SF,
Take a look at the Hammond 290FX
It's only 320-0-320, but the thing is a beast at 518ma   (you could install central air conditioning in your next build  )
It's got the bias tap and 5.5A @6.3
At 122VAC input, I've got 470VDC coming out of the rectifier, and with 4-12ax7s and 2-EL34's it doesnt even blink

Only problem with it is it's a laydown stlye, which is a deal breaker for some:
http://www.hammondmfg.com/pdf/EDB290FX.pdf

Wow, I just checked $ and someone over at Mouser must have bumped their head:
http://eu.mouser.com/ProductDetail/Hammond/290FX/?qs=%2fha2pyFadugRw8pp2c3tUgo8j3jjFF9PxJ6X8foAC6I%3d

OK, I just realized i was looking at the Europe site....damn Euro's

[silverfox]

Flash- Silverguns Tranny just came in. Looks like what I have but I never checked the HV current rating... I'll look into it. I checked one yesterday and it was quite expensive. May have to rethink the separate bias winding.

Please, correct me if I'm wrong. I don't have any info on this transformer but what I could determine. It could be similar to an Ampeg tranny set. These came out of a 4 channel PA running 4 preamp tubes and 2 7868's. Roughly speaking, a rectified B+ at 420 vdc and 100 ma is: 40 watts total dissipation. The preamp and others should draw minimal current.

I have gotten 2 EL34's and 1 _AX7 to work on a power amp basis but don't know at what power level.

My plans are to use 3 - 4 triodes (2 triodes/tube) in the preamp stage, 2 for an overdrive and 2 for an effects loop and two in the pi. I would also like to experiment with a 2 input mixer within the build.

Think of it as a government funded project...

Silverfox

[labb]

Handy thingy for calculating PT current requirements>

http://www.dreamtone.org/Calculate_Current_Form.htm

[silverfox]

labb- Thanks for the lead. I've been there before but hadn't seen that page. The specs I put in were 5 preamp tubes, two EL34's and that came out to about 56 watts.

Is there any real benefit to insisting on a separate bias winding or should I just scrap that idea? It seems to make the tranny a lot more expensive.

I believe what happens if an undersized power transformer is matched with a higher value output transformer is the system just peaks at the available power. It wouldn't burn out would it?

Fox.

[silverfox]

I just answered an add for a Univox 1221. It powers 2 6L6GC tubes, preamp, tremolo and reverb tubes. I'm going to take a look at it. Probably just needs servicing.

Makes sense if it works at $100.

It's sad what has happened to the industry, the economy; I was explaining to my wife that this amp a couple years ago could have sold for $200- been modded for another $50- and then turned around and sold as a real screamer for $500. try that in my area now and you'll end up with another addition to the collection. Since that's what I'm after, projects, it works for me.

Silverfox.

[SILVERGUN]

I believe what happens if an undersized power transformer is matched with a higher value output transformer is the system just peaks at the available power. It wouldn't burn out would it?

Let me be the first to say....that is not a good theory 
Yes, you can/will burn out an under rated PT regardless of what OT you use
The OT just gives you a connection between the power tubes and your speaker....it doesn't limit current through the PT

You don't need the separate bias tap.....it is fairly common for some amps to knock down one leg of the HT for the bias supply
 

[silverfox]

I bought the Univox 1221. It looks like about what I'm looking for in parts and I got the whole amp for $100.

Here's the schematic: http://www.freeinfosociety.com/electronics/schematics/audio/univox1221.pdf

It's perfect in many ways for a build or mod. The cabinet looks cool and the chassis is just right for where I'm going with my building experience. I played it for a while and it sounded okay. Didn't over heat the trannys. Lots of hum due to the 2 prong cord and need for a recap.

For now it will sit. I'll fix the ground hazard and mess around with it for awhile. I don't want to start anymore projects until I'm done with my power amp. Then I'll query the forum for suggestions on mods or what. I think this could turn out to be a really cool build.

How may watts could it be? Says 100 but that has to be total consumption since there are only 2 6L6GC power tubes in the output. Sounded like 18 to 25 watts to me in the garage but there's no impedance data on the speaker jacks so it could have been mismatched.

5 preamp tubes so the trannies are just what I wanted. Got me to thinking of started a salvage yard for amp part$.

Silverfox.

[PRR]

> Says 100 but

Barely possible.

660/620V on the plates!! This is similar to 6550's 100W condition. The 6L6GC's Pdiss is in-sight of 6550's Pdiss. It could really run 100 Watts out until the 660V kills the tube. And we know classic 6L6GC plate-stuff would take 600+V.... the question is the socket and tube-base.

Personally I'd switch to 6550. And close-examine the sockets, replace with best-quality Ceramic if any suspicion of arc-over.

That era UniVOX are wonderfully well-built. Yes, the caps are not getting any younger. Interesting just five 350V 47u caps will do the deal. I'd fix the leakers, spray the pots, wax the finish, and party like it's 1973 all over again.

[DummyLoad]

>>That era UniVOX are wonderfully well-built. Yes, the caps are not getting any younger. Interesting just five 350V 47u caps will do the deal. I'd fix the leakers, spray the pots, wax the finish, and party like it's 1973 all over again.

they are a  classic, just not highly regarded. i'd re-cap and shine it up too.  

bond norm. ch1 to brite ch2 with short patch cord and plug into brite. ch1 - that should wake things up.

cool! 6AN8 triode is CF driving the reverb tank. stolen! ;)

>>there's no impedance data on the speaker jacks so it could have been mismatched.

link to schematic you provided says load is 16 ohms connected to the OT sec. white wire - 8 ohms to the green... 

output stage is reminiscent of music-man output stage that has 700V to plates only no SS current source. if new finals are needed, you could use sylvania 6L6GC STR415 that are still available.

--pete

[HotBluePlates]

660/620V on the plates!! 

Yeoww!!

output stage is reminiscent of music-man output stage that has 700V to plates only no SS current source.

My feeble understanding of the MusicMan amps is that they are painfully close to class B (if not in fact idling near zero current, to the extent any tube can really be run that way). That these amps don't use the output tube control grid but are essentially driven by solid-state devices attached to the cathodes and controlling tube current.

I see the MusicMan amps have a reasonable class AB bias on the control grid (as does the Univox), though it's not clear to me if the devices in the cathode circuit have the idle current clamped low before applying a drive signal. The Univox doesn't have this extra leverage on the output tube current, and so the 6550-like output voltages are surprising.

Also surprising to me is the voltage doubler supply for the 6L6 plate voltage; I wonder how deep that sags with an actual drive signal.

[silverfox]

HotBluePlates said- "Also surprising to me is the voltage doubler supply for the 6L6 plate voltage; I wonder how deep that sags with an actual drive signal."

Perhaps that's what I heard at some points in testing. The tonal quality dropped out, not in a real good way and when I plugged an amp emulator in, it seemed to overdrive the amp in an unpleasant fashion- even when the effects unit was turned down. By itself and at about 1/2 power it did sound good and even at close to full volume. However, at full volume, as I recall, changes in the tone control seemed to weaken the output.

That was not a concern for me since I was only after the Iron at the time. I'll probably invest in improving it as noted in the Topic I started.

Fox.