• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Newb seeks to drive DVC 4+4 ohm 500w RMS subwoofer with tubes.

Not open for further replies.
Greetings tubemongers!
Your contributions to the world of tube topology are appreciated by lurkers everywhere.
THis is is intro/question. TLDR;) is at the bottom ;)

AS a boy I was in LOOOOOOOVE with speakers. LOOOOOOOVE!
Having no resources (OR REFERENCE MATERIAL) I experimented, taking 6x9s from a junk car for example LOL. With these and some 5gal buckets and pipe I designed something akin to a sixth order bandpass at age 14 :eek: wow it sounded great (as my urbanite cousin rolled on the ground laughing because his father had given him a Marantz from the 70s)

Then there was the suitcase PA I got at a rummage sale for $5. My first tube device. Complete with pair of 6" CTS alnicos it was the best woofer setup I'd ever heard, even impressed my cousin.
Unfortunately it was lost over time after becoming extremely hot during operation (I had no idea what a tube was or how caps worked)
I did keep the speakers but no other amplifier EVER reproduced the original sound.

Now I came here via the world of guitars. My starved 12AX7 modeller needed a new tube and I bought a small lot of used of Epay.
The testing went... meh... meh... what the.....HOLY CRAP!!! WHERE HAVE VINTAGE TUBES BEEN ALL MY LIFE?
Ironically the winner wasn't a fancy Mullard, Amperex, Ken-Rad, but a short grey plate RCA from the late 60s.
:whacko: stricken :whacko:

Now I've been collecting old Hammonds for parts and have managed to get a good stock of components.
Just last week I got something with 24x RCA long black plate 12AU7 :D
Hopefully a lifetime supply!

The problem comes in finding the specific properties of any given OPT.
I've only built things (not tube circuits) from new or known components.

Would you please give me some advice on building a 100 +W tube amp to run a subwoofer?
I know that's a vague question. Unfortunately I'm half pro/half newb :shy:

My current list of components are basically standard audio tubes EL86, 12AU7, 6V6, 6SN7, 12AX7, 7591, some rectifiers 5GB4, GZ34, 5Y3 ?
All the speakers were 8ohm wired except a HR-40 amp which runs 4 x 6V6 into a 2 ohm load. It was only advertised as 20W output though.

I picked up some 25DN6 and a couple others from the TV tube thread but no sockets yet.

Advice or a proven circuit?
Speaker can be wired either 2ohm parallel coils or 8 ohm series coils.
That whole thing is TLDR :eek:

What features (other than a LPF) favor 20hz-200hz?
How do I determine the limitations of a particular random OPT?

Please don't think I'm being lazy. The TOOBZ for NOOBZ thread is full of so much abstraction and the internet has become so crowded that the search might soon be useless.
I'm self taught which means I had an incompetent boob :D for a teacher :D
I guess my question is: Why would you wish to run a sub with a tube amp? All my gear is tube, except my sub. I replaced the cheap Chinese SS class AB1 amp in my Monitor Audio sub with a Hypex UDC 400 class D amp module & switching PS. No noise or hum, clean well defined bass, little heat and it just works every time the system is turned on.
SS is the logical choice John and I do own a couple rack amps.

Since starting messing with tubes I've experimentally powered this sub off several 2 x 6v6 amps (Hammond AO-29 etc) and it always sounds "better" than SS. (NOTE I didn't say louder)
I don't know what the difference is but I know what I like.

I'm not looking to use it all the time.
There are also two full stacks of 12" alnicos which sound better with tubes.
That would be 16 X 25w + guitar use = ???
Stocktrader, good point on the HZ rating. Exactly the kind of things I need help with and that point is on my radar now.
I understand how hz effects motors and what that has to do with power transformers. Is there an explanation somewhere of how this works in OPTs?

Now I'll be a bit more specific. The best mains transformer I currently have came from a device running....
12AU7 x 18
6C4 x 1
6V6 x 2
12AX7 x 2
6BA6 x 2
12BH7 x 3
6BJ7 x 1
5U4GB x 1

B+ in the original device is around 325v, 120vac rating 160w

Seems like I had one larger transformer, almost a Kleenex box.... off to dig around...
Yeah that Champ 1000 is interesting but a little ... well it's not a little.... you know....

EDIT maybe I should mention that I'm willing to work within the confines of this power transformer's capabilities.
I'm mostly looking for ideas and a little help putting puzzle pieces together without being so specific as to drive the thread into the ditch.
Last edited:
OH...... I must have deleted that part about wanting to make 100 or so watts off sweep tubes.... speaker is 88db@1w, not the most efficient and it'll take a little juice to get it moving.

yeah that would be 160w @ 120Vac :D
I can do the math given a formula, but have no reference. (plus there are a LOT of formulas and picking the right one is harder than working the maths)

I do know how to calculate the heater currents and convert that bit but I'm a lost on the actual ac-dc-ac-speaker bit.......
reading around here doesn't help much http://www.diyaudio.com/forums/mult...much-voltage-power-do-your-speakers-need.html
2.9? What?

I don't like sounding 16 years old but it happens when you try new things..... :bomb:
It doesn't help that I can't edit posts after 30 mins (for the purpose of not sounding completely &#@%)
Yeah it hurts :eek: I do a LOT of things but EE is not one of them... repairs yes... engineering no....

!!!! I'm really tempted to ask a mod to delete this thread and try later with some simple specific questions !!!

What the heck am I supposed to do with cantaloupe sized mains transformers from tube powered organs?
I can re purpose the other parts to guitar amps, what about the 17lbs main?
Last edited:
you will need at least 750 VA power supply transformer for 500w (peak) output as music does not average near max power. bass does require more power as it sustains with bass guitar and peaks for kick drum. In a sub situation it may also be set to play 6 -10 db louder then the stereo signal.
transformers can not pass any DC, so as the frequency gets lower their ability to function effieciently is reduced. inductance and core saturation become problematic.
I used a 70v line tranformer to output my first tube amp project, above 100HZ 15 w is easily reached. at 50 hz 5w is becoming distorted by core saturation.
the bigger the core, the lower the frequency limit for a set power level
Getting to 100 Watts output with a tube amp is quite do-able. Edcor has the CXPP100... range Output Transformers for around $100.


They also have power transformers and power supply inductors.

Using TV Horizontal Sweep tubes (instead of the usual "audio" tubes) for the output stage can greatly reduce the price also.

Some cheap, but quite decent, TV "Sweep" tubes: 6HJ5 ($5), 26DQ5 ($3), 6CB5A ($5), 21LG6A ($4), 21JV6 ($3), 6EX6/6CD6 ($5), 36LW6 ($11)

You can get to 200 Watts output by using two 100 Watt OTs, either in parallel, or by going to the Crowhurst Twin Coupled approach (an OT in the plate circuit and another OT in the cathode circuit).

Industrial voltage change (like 120V to 440V/480V) transformers (off Epay) can give you an inexpensive power supply.

This DIY thread has covered 250 Watt amplifier versions using such components. (long thread, somewhere in the middle, posts by Tubelab.com):


Above 200 Watts output, the going gets expensive fast. And REALLY dangerous HV. Much more sensible to split up into multiple 100W or 200W amps with more than one speaker. Redundant against failure mode that way too. (public events, parties...)

And then there are OT-less trick designs (save $$, but more complex) that use a tube/SS hybrid output stage. Use a big "Sweep" pentode to drive a SS bipolar or Mosfet in Darlington circuit configuration, with speaker output voltage fed back to the pentode Screen grid(s) (with a series +120VDC boost supply to each screen grid). This makes for a "power triode" emulating output stage that can drive the speaker directly (typically in a Circlotron style P-P circuit). Requires two +48V B+ supplies for the output stage and a cheap Acopian (or similar) regulated +250V supply for the front end tubes (use cheap Epay stuff again).
Last edited:
Another OT approach (for low frequency) would be to use a big industrial 120V:660V voltage changer xfmr. (like a 1 KiloWatt unit)

Use big TV "Sweeps" in parallel to drive the 660V winding in Circlotron mode. Use a floating +480V B+ for each bank of tubes. The 120V winding becomes the speaker output. For a 30 Hz response, the xfmr voltage ratings are halved. The resultant 60 Vrms output rating is still good for 450 Watts out at 30 Hz (for an 8 Ohm speaker, 900 Watts for 4 Ohm). (or 200/400 Watt at 20 Hz for 8 Ohm/4 Ohm)

A 50 Hz rated xfmr would do a little better yet.
Last edited:
Thank you very much Mr. SmokingAmp!

I have been stalking Mr. TubeLab's posts.... you're next! (where is my Muhahaha smiley when I need it? ;) )

Would you please tell me a little more about determining the correct characteristics for this OPT? Is there some ratio I'm after?
I'm only aware of 50hz and 60hz primary windings. Do they go lower?
I have a 12kv neon sign tranformer (kidding)
Normally, 50/60 Hz industrial voltage change transformers are short on inductance (or turns) for tube output transformer use (at low freq. like 20/30 Hz). But they can be de-rated voltage wise. 30 Hz usage will allow half the voltage rating of a 60 Hz rated unit. 20 Hz allowing 1/3 of a 60 Hz rated. Magnetic saturation will occur above those voltage limits. The current rating of the copper windings stays the same of course. A 120 VAC winding turns out to be right in the ball-park however for the speaker secondary at these higher power levels. Watts out = Vrms*Vrms/Zspk
So 60 VAC out (at 30 Hz) gives 450 Watts with an 8 Ohm load, or 900 Watts with a 4 Ohm load.
Or 40 VAC out (at 20 Hz) gives 200 Watts out with an 8 Ohm load, or 400 Watts out with a 4 Ohm load. That will take around a 1 KW transformer to handle the current through the copper windings.
(to avoid a poor speaker damping factor) About a 60 Lb xfmr.

Hi-Fi usage normally requires the primary winding reactance XL to be above the primary Z to keep primary magnetizing current under control in the audio passband. (XL = 2 pi f L)

Using some beefy TV tubes (high current capable) with a "local" feedback scheme to lower their output impedance, Z, can overcome this inductance issue. But not the AC voltage de-rating issue. So AC voltage is the key limiting factor.

Not much xfmr stuff (aside form utility co. and specialty HV xfmr stuff) around above the 660 VAC rating. But worth doing some searching. You normally need around twice that rating for tube stuff in P-P (using a traditional center tapped B+).

The Circlotron circuit comes to the rescue here, since it can work with half the primary voltage rating (each P-P parallel tube bank uses the SAME winding). It does require two floating B+ supplies (around 480V at this high Wattage) however. But industrial xfmrs come to the rescue again here.

The Circlotron circuit requires abnormally high grid drive voltages to the output tubes, since the cathodes are handling 50% of the voltage swing (50% CFB). This difficulty can be overcome by using bootstrapped loads on pentode drive stage tubes that are rated for HV. (just use some small TV "Sweep" tubes for the driver stage typically, like 6JN6, 12GE5, 6GF5, 6AV5, 6AU5) (can get 6GF5 for $1)

One other possibility are Constant Voltage xfmrs. These typically have a 600 VAC winding on them for the resonating capacitor drive. By removing the magnetic shunt bars, one would end up with a 120/240 VAC to 600 VAC xfmr. An un-explored approach. But if the local junk yard has a 1 KW one for cheap (with 120 VAC primary), could give it a try. (usually SOLA brand, but there are others)
Last edited:

A 500W valve subwoofer amplifier goes way beyond my definition of pointless,
as it is such an awful idea, and has so many drawbacks, especially long term.

Whatever advantages their are in the "sound", it could be replicated by
careful design of an input stage and loaded transformer coupling to an
output stage, there are far more sensible options than not sensible.

A loaded valve based buffer to the sub amplifier makes far more
sense than this madness. Class D is the obvious output choice.

rgds, sreten.
Yeah, 500 Watts output is going to get one a 250 LB amplifier, with 1 KW OT and 1 KW power supply, power inductors, heater supplies for parallel big "Sweep" tubes and a strong case. Going to need a fork lift to move it around. Not going to be useful for portable applications for sure. You won't have to worry about anyone stealing it however.

It does make better sense to use several smaller amplifiers if one really needs 500 Watt. Reliability, portability, configurability.

But sometimes it's just one of those "climb Everest" DIY things.

I hope those speakers have ferro-fluid cooling and forced ventilation. 500 Watts is going to do some damage to most houses. Certainly will shake the beer bottles in the "man cave".
Last edited:
Sreten you objection on the grounds of impracticality has been noted.
As for the rest of it I have no idea what you said. It sounded good though! :D

What is "Z" ? (google laughed in my face when I asked)

Circlotron! Sounds like a awesome name for a warhorse!

Wait... Circlotron is output tranformerless? They don't LOOK like a monstrosity ??? :confused:

I'm reading your post and searching.
No, Circlotron is just an uncommon type of OT/tube configuration. Its actually quite high performance (high bandwidth) compared to the usual center tapped P-P OTs, but rarely used. ElectroVoice used it. Out performs Mac.


Z in amplifier speak is referring to impedance, similar to resistance, but more general to include some inductive or capacitive reactance.

Output Transformerless:
This is another approach that can produce high power using battalions of tubes in parallel. And it saves on the cost of an OT. But generally it is quite inefficient. You would need a dedicated air conditioner to remove the heat for a 500 Watt amp. And it is known for eating tubes and sometimes speakers too. I would call it the epitome of pointless.
Last edited:
Not open for further replies.