Sanity check my huge amp idea?

[92Volts]

Hey all! I'm new here and hoping to learn... I like tube gear and building amps, but I really love thinking over new designs. I hope you guys can help me with my weird ideas and maybe I can give back to the community with what little knowledge I have 

I found this 750w residential power distribution transformer for 480v to 120v conversion. Naturally, I want to run it backwards, to make the B+ supply for a massive tube amp. OK, sanity check's not the right phrase, obviously not sane!

But I had some questions about output stages. I'd actually have 2x240v and I was thinking of rectifying and stacking them to get about +325v and +650v DC.

My first idea was stereo with PP EL34 for each channel. This seemed perfect since EL34s die at high voltage... but research indicates screen voltage is the real culprit, and I can set screens to 325v.

But EL34s need high impedance at that voltage (8k at least). Edcor offers OPTs with those ratings, but I'd like this to work for instruments or hi-fi... and I found reports that Edcor's high-impedance models distort treble: https://www.eevblog.com/forum/projects/calling-all-tube-heads-recommend-me-an-alternative-output-transformer/ 

The expense/difficulty of finding EL34 OPTs for that voltage brought me to KT88s. I'd dismissed these because I thought my screen tap would be too low... EL34 datasheets are the only I've seen that give explicit examples for running screens as low as 1/2 plate voltage (800v and 400v).

But then I found talk that KT88 screens should be kept lower than people realize for better tube life. Maybe 325v is perfect.

So here's my idea:
Stereo with one pair of PP KT88s each side, 650v on plates and 325v on screens. Fixed bias, expecting 100w or more per channel.
Output transformers: one Edcor CXPP100-5k per side. This is slightly higher impedance than datasheets recommend for 560v. That seems OK, but apparently lower screen voltage might call for higher load impedance? That's a concept I don't understand. I thought high impedance mostly served to avoid high plate dissipation, and I don't understand how it relates to screen voltage.

This amp would weigh at least 70lbs 

Any ideas? I'm open to totally different plans, including wiring the transformer for a single 325v supply. However, I don't see obvious ways to use significant power at such a low voltage. I'm also open to giving up, because this is silly. But I'm having fun planning it out.

[sluckey]

Weld a chain to it and put it on your boat.   

[shooter]

Weld a chain to it

+1

I did a stereo KT88 build, basically 2 mono blocks on a single chassis  it weighed >50lbs, worked fine.  It has since became a donor box for other builds that were way more practical. 

[92Volts]

I'll keep that in mind... I'm already planning a 2-channel preamp for instrument/hifi so boat anchor seems like an obvious third use!

I'm thinking I'd rarely move this amp with my current plans (I don't expect to play live music soon) but I'll definitely be considering the implications.

The HV transformer I have weighs 20 pounds, those Edcors are 10 each, plus filament and/or bias transformers depending on how I handle those needs. Then everything else. 50 is the absolute lowest weight it might be.

[PRR]

> lower screen voltage might call for higher load impedance? That's a concept I don't understand. I thought high impedance mostly served to avoid high plate dissipation, and I don't understand how it relates to screen voltage.

I do not see the word "current" (except later in "current plans").

Power is Voltage times Current.

Plate supply voltage dominates plate voltage swing.

G2 voltage determines the maximum *current* a pentode will pass.

It is usually awkward to have separate supplies. Most "audio power" tubes are scaled so Vg2 may be "near" Vplate, and the tube will make good output power with a low-cost low-impedance load. Small power tubes will be happy with 250V-300V (plenty of margin on 450V supply caps, even if you have a field-coil loudspeaker). The larger tubes may be happy with 400V-450V.

But if it is convenient to have *separate* supplies, you may increase Vplate. Since the max power is still limited by dissipation, you do not need as much *current*. You may reduce Vg2. You should reduce Vg2, for less Vg1 drive/bias needed, and less stress on G2 when over-driven. 6550, KT88, and 8417 all show conditions of about 600Vp 300Vg2.

> I found reports that...

Don't believe everything you read on the interwebs.

The problem of wide-band audio transformer treble response does get harder as impedance goes up. Any 1K winding will cover the audio band. A 10K winding with enough inductance to cover the bass will have more leakage inductance, they tend to scale together. However the stray capacitance does not scale, goes pretty much by winding-lump size. So at some impedance the leakage L and the stray C will resonate at the top of the audio band. There are ways to reduce this. They add cost. They were developed near 80 years ago. Edcor is a newer outfit, doing well supplying lower-Z utility transformers, and may not have all the old secrets at hand. (However I know critical studio engineers using Edcore iron and not dissing the treble.)

Remember you are reading this on the interwebs and it may be bogus.

[shooter]

using Edcore iron

fwiw I've had great luck using Edcor, I've owned 5 OT's, 2 currently, and weather it's Fi or guitar, they shine. 

[92Volts]

Thanks PRR! You bring up a few good points and helped me understand better.

I knew screens influenced maximum current draw but I didn't fully understand why it mattered in "normal" conditions.

From what you said, and another look at datasheets, it often doesn't matter-- as long as it allows the amount of current you'd want to use to stay within dissipation limits, etc. Generally, it sounds like you can't arbitrarily lower screens... but once they're enough for the tube to operate normally you can arbitrarily raise plate voltage without raising screens (within tube limits).

I try to be skeptical when reading stuff online, but you actually provide your own explanation for why you would expect (or believe) bad results from high-Z Edcors. I don't think this makes them a bad company, it just shows it's easier to find good 5k transformers than to find good 8k-10k transformers. Between shooter's testimony and others I do plan to buy Edcor iron, I just want to avoid the need for high-power high-Z iron

[PRR]

> you can arbitrarily raise plate voltage without raising screens

_IF_ you also raise the load V/I (impedance) in the same ratio.

I think you have this idea, but I'll leave it for other readers.

[drgonzonm]

Weld a chain to it and put it on your boat.   

I do not think Franco would receive this type of comment.   My guess the transformer weighs in about 10 lbs. 

At 70lbs, make sure it has wheels and a handle. 

I would look closely at how the the xfrmr is wired.  It might have dual 120v outputs, (i.e. look for a center tap on the secondary. it offers you some more options, especially if center-tap wires are separate. 

[silverfox]

Until you spend a little time studying this Rolls Royce of audio amplifiers, I don't think you realize what a "huge amp idea" is. Hope this helps a little. This amp purports to do everything but bring you the Newspaper. Lots of good ideas and the design is, I believe, tested.

At 19 tubes, Behold Dogzilla!

http://dogstar.dantimax.dk/tubestuf/dzindex.htm

silverfox.

[drgonzonm]

Large amp,  I found a website awhile back that showed a picture of a bass amp, whose OT weighed 85 lbs,  It had pneumatic wheels and a handle to move. 
Power tubes were not so tiny, likely transmitting tubes, but I don't remember seeing plate wiring on top of the tubes.   

[92Volts]

Thanks again to everyone!

drgonzonm, the transformer has 2x fully independent 120v "secondaries" (I'd use as primaries) and I considered using 1x 120v winding to alter the voltage of the 240-480v "primaries" (I'd use as secondaries). The resulting 360vAC would give 500ish VDC which is ideal for many designs...

But I worried about overloading the remaining 120v tap, the only remaining "primary" in this application.

Transformer rated at 750W... assuming ideal efficiency (70% in class AB) and pushing all limits, KT88s dissipate 42W per tube while a pair delivers 2x dissipation to the load (168W output per side)... this amp could want as much as 504W to the plates (plus preamp, etc). I don't want to re-purpose 1/2 of the transformer's "primaries" when more than 1/2 its capacity might be needed.

Then again, this is a heavy residential power-distribution transformer and it's likely expected to provide near zero voltage sag, inefficiency, etc, within rated capacity-- if so, maybe it's OK to exceed the limits of one 120v winding by 30%-40%, especially since it would be at peak power only, and it's still within the overall power rating of the transformer?

The transformer could definitely provide 240vAC (325 vDC or so) at full power, but I can't think of a relatively-sane way to use so much power at low voltage.



silverfox, thanks for showing me Dogzilla. This is both an example of what I want to build, and an example of why this is ridiculous.

I find it interesting that 1x 100w Hammond output transformer might handle 200w output. This opens up a variety of other, simpler, cheaper configurations to me, but stereo is still appealing because it could serve a dual purpose as a hi-fi amp. Dogzilla was also built on a surprisingly small chassis... before seeing an example like that, I was worried about finding a big enough chassis.

Drilling holes and other non-electronic aspects of this project are starting to scare me... the Dogzilla writeup shows how much might be required for such a large amp.


PRR, thanks for the reminder. I was aware of this, but load impedance will be vital for both me and anybody else considering high-plate-voltage builds in the future.

[PRR]

> as much as 504W

No; but whatever.

Where the heck are you going to play overdrive guitar at 500, even 300 Watts??

This fails the Sanity Check.

[silverfox]

As an aside, this is a very dangerous transformer if not properly respected.

I remember an experiment I conducted using a Console Color TV flyback transformer, and a Radio Shack Coil-Flash-Tube set. I manually triggered the flash using a screw driver to switch the power. Because at the time I did not do the math, the flash tube arced and burnt out. During the course of further experiments I failed to realize the circuit was energized at 700 volts. Fortunately nothing happened but I never knew the circuit was energized until I was done messing with it.

Be careful.

PRR: "Where the heck are you going to play overdrive guitar at 500, even 300 Watts??"    On the Rim of the Grand Canyon. But You'd get a ticket and have to go to Federal Court!

silverfox.

[DummyLoad]

This new KT120 is the steamroller of audio tubes, prepare to be flattened.

https://www.tubedepot.com/products/tung-sol-kt120-power-vacuum-tube

lol!

--pete

[drgonzonm]

I believe you can run the transformer using the two 120's windings.  You gave some key information, "two independent"

 Say you label the secondary wiring as x1, x2, x3, and x4.  to operate as 120-0-120 x2 and x3 would normally be tied together to give you your neutral/ground, and x1 and x4 would be 180° out of phase, just like the power going into your house.

Now if you tied x1 and x3 together and tied x2 and x4 together, you now you can fuse those converted secondaries with a 12 amp fuse have about 1500w to feed those hungry power tubes.  Who need class D amps anyway.

Now with your filament transformer, you probably with need a 15 amp fuse/ say a 1/4 din circuit breaker on your amp.  (time to go to harboor fr8, and buy one of those four wheel carts to mount the amp on.  (Low tire pressure to minimize the bounce)). 

Now where do you find a nice little OT for this project?   4,6, 8 kt120's? Resistor or choke? Motor application caps?  what tube(s) do I use for the PI?  Horns or speakers?.  I don't think Doug's boards are heavy duty enough! Decisions, decisions. 

Who says I need to use a DI box and tie into the PA system. I have my own tiny stadium amp. 

I hope you build the amp, and post pictures. 

[drgonzonm]

This new KT120 is the steamroller of audio tubes, prepare to be flattened.

https://www.tubedepot.com/products/tung-sol-kt120-power-vacuum-tube

lol!

--pete

I ain't afraid of no volts, (to misquote a ghost buster).  Use transmitting tubes at 3000v, and save the KT120's for the preamp.   

[shooter]

Use transmitting tubes

I have a couple left over from my MRI days, 25kv plate, pulsed 10% duty cycle, weight ~5lb, you'll have to do the math for SE operation

[drgonzonm]

Use transmitting tubes

I have a couple left over from my MRI days, 25kv plate, pulsed 10% duty cycle, weight ~5lb, you'll have to do the math for SE operation

what are the specs, maybe they will operate at 500v

[shooter]

the specs

both pre and PA are Eimac, parts numbers;
3CX800A7
YC-156-A
they very seldom failed, I just changed them when the clock hit 20k hrs, or bias couldn't be set within OEM specs.  currently they set in the barn a curiosity pieces

[PRR]

> maybe they will operate at 500v
> 3CX800A7

3CX800A7 at 500V and zero grid will pass maybe 50mA. Bah, 6L6 or 300B will do this.

To get any real power we need 2,000V on plate, force the grid 24V positive where it will suck 150mA. 4 Watts peak drive in 160 Ohms. For that we may get 300 Watts in SE class A2. 5K 400mA SE OT.

Power gain near 100, only about 20W heater power. It is a good tube, just not convenient for audio.

YC-156-A is a bigger tube.

Both need howling fans.

[drgonzonm]

> maybe they will operate at 500v
> 3CX800A7

3CX800A7 at 500V and zero grid will pass maybe 50mA. Bah, 6L6 or 300B will do this.

To get any real power we need 2,000V on plate, force the grid 24V positive where it will suck 150mA. 4 Watts peak drive in 160 Ohms. For that we may get 300 Watts in SE class A2. 5K 400mA SE OT.

Power gain near 100, only about 20W heater power. It is a good tube, just not convenient for audio.

YC-156-A is a bigger tube.

Both need howling fans.

Howling fans, NVLV, or New Mexico Lobos?  (oh yeah, cooling fans ) 

Only SE, why not PP, got to put some those audiophile triodes to good use.   (300-400w, maybe an 85 lb OT isn't large enough )

Voltage doublers?  (don't forget to account for the x-rays, and set up the amp in the next room, line the wall with lead). 

Convenient for audio?  This amp ain't going to be convenient for anyone except the power company.

Yes, sanity is also out the window. 

[shooter]

don't forget to account for the x-rays

I might have some sheet lead left over from my CT days
you use the YC you might need some RF shielding or an FCC license

EDIT:
I did use a 3C.. tube a couple builds back, worked great........as a heat sink/magnetic flux interference tube

[DummyLoad]

> as much as 504W

No; but whatever.

Where the heck are you going to play overdrive guitar at 500, even 300 Watts??

This fails the Sanity Check.

I'd like this to work for instruments or hi-fi...

he did say Hi-Fi! :p   

sowter OT - 200W 2 pairs of 6550/KT88 -or- 400W 5 pairs of 6550/KT88 in pentode mode.

http://www.sowter.co.uk/push-pull-output-transformers.php 

you'd want a pair of U71 200W or a single U72 400W. the freight is probably a small fortune itself. the OT are $450 each 200W or 400W same price.

locally grown (canadia eh?) hammond 280W 1650WA - 280W - $348 each.

--pete

[Ritchie200]

92 Volts, you did ask for a sanity check so here is my unfiltered .02....

Weight: Take a look.  4 KT88's, iron, chassis that can support the iron, parts and cab.  I always asked for help hoisting it up on a full stack.  Expensive box teetering on top.

Usefulness (or Uselessness, based on your view):  In over 10 years of playing out with this amp, I was able to open it up without two Altair power soaks hooked up to it exactly three times - two outdoor shows and once in a big auditorium where acts like Sabbath and Zeppelin played.

Power output/Tubes: Two KT88's will NOT get you 168 watts (RMS). Two pristine REAL Gold Lion's from days of yore would not get you there.  Ever. Never.  Ever.  Four KT88's at a voltage new tubes will live at (south of 600v), with screen voltages they will live at, might get you 168 watts.  KT120's may give you 120 watts for a pair and will live at higher voltages, but higher voltages will require components rated for that realm of operation.  You think iron is expensive?  Buy a few quartets of GOOD tubes (a consumable item) and check your bank account.

Seriously, I mean seriously?: If your desire is to build a one off steam powered fire breathing Clem Cadiddlehopper special that will tie your shoes and wash your car at the same time - more power to ya.  But even then, why?  So you can post some pics on the internet of a compromised design that does not work well for either of your intended applications?  That's an expensive road for bragging rights.  Then what will be a true boat anchor, will end up collecting dust on your bench when you are done. 

Reality?: If you are interested in a high power design, why not look to proven designs, at acceptable voltages for the tubes, and buy iron suited for the job?  It will be cheaper in the long run and you will have something you can actually use.  Just one example, you mentioned that you were surprised that one of the cheaper lower wattage Hammond OP trannies would work.  For guitar, maybe.  For HiFi, nope.  There are a couple of guys here who have built several amps using KT88's and KT120's.  You could collaborate with them and come up with something you can be proud of when you are done.

Please do not be offended.  I completely understand the desire.  I am the Supreme Commander and President of the stupid big and loud useless amp club.  It is my choice, yes I have a problem, and I am not looking for help.  However, unless you are looking to replicate the sonic assault of a certain precious model from the past, put your money and effort into something you can actually use.

JMHO
Jim