• 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.

I'm new here and I'm building 4XEL34 based amp.

how is your project doing?

Hi Im a musician from denmark , have build a lots of small diy projects,
And I was wondering how much similarity there is between the output stage off a marshall ,like the one you build, and the old legandary Ac 30 (wox)

Because I would like to make up a amp like the AC 30 and maybe modify the preamp section a bit.

But up to now I havent get startet it seen to me that the audio trafo is not that easy to get for a quite low price.

do you know of good supplyer (europe)

Since that I don't know very much about how to disign the circuit my self but like to follow already made circuits , Ill be happy to get some good circuit for an 50 Or less w tube amp for guitar.
like if it has a good preamp section with drive the can be sw-in.

hope to hear from you.

jc. dk
4XEL34 Marshall guitar tube amp

Ultralinear: This one is for guitar.


There is a lot similarity between Marshall 4XEL34 and VOX AC30. There ain't so big differencies in any of those 4XEL34(or similar tubes) schematics.

I'm lucky to have a friend that works in transformer factory, so there's my transformer source at this time. But it aint' still that easy for him to do those transformers. So I can't help you straightly, but I suggest you to call local tranformer suppliers and ask them. Believe me, they know how to calculate impedancies (and take consider voltages aso). BTW, my audio transformer will have 1,7 kohms (with center tap) primary and 8 ohms secondary with center tap for 4 ohms.

I'm in same situtation with you when it comes to tubes- I really can't design them yet. I know principles and can modify them, but I'm not ready to design them, so I will also follow ready made schematics.

In thought I'm really interested of those Air coiled OT's at the moment. If I just could find a little bit theory of them :)
HEY!! Just a discussion I was looking for!!

I'm up to building a Orange Graphic MKII guitar amp, and this uses 4x EL34. I where (until recently) very unsure about what Prim Imp to use, but has decided on a hammond Iron with 1,9K prim and 120W power handling. This Amp gives will give out ~120W.
Another version uses only 2 EL34's and gives out 80W. I've heard some place that the 2 versions used the same OPT, but I've heard the opposite too!!

|There is a lot similarity between Marshall 4XEL34 and VOX AC30|

As far as I know, the AC 30 uses 4 EL 84's (4K prim) wheras the AC 50 uses 2x EL34 (3,5K prim).

Do you know any commercial Irons with a 1,7K prim?

Good pages for info are:


Duncanamps have this very nifty program called "Tone Stack Calculator". It has a number of filters from well-known guitar-amps, and lets you mess up the values, and then you can check out the freq. response! Real fun!
What I like about fidling with guitar amps, is that you can use all the crappy tubes not suitable in audio! I'm planning to use the Steve Bench filter from TSC with a 6J4, 9002 and a 5879. Great fun!!
Zero bias of tubes gives exelent overdrive fuzz BTW!

Yep, that's true. 4XEL84 no EL34 !!

Yes Stigla, that is very true ! In Vox AC30 they have 4XEL84. But what I ment was that when we're in same class, schematics with 4 tubes are quite eguivalent.

What comes to primary of OT's, IMO it's very easy to calculate it. All you need is the Datasheets of this certain type of tube. If you just deside to change primary impedance, it may damage you tubes. Not that, that I'm expert in this subject, but just advise. These guys in here DIYaudio would propably know MKII OT's primary impedance ??

I don't know any commercial 1,7K prim. OT's. Unfortunently, because I would've propably buyed one myself.

I chekced those programs, cool ! I'm amazed that I haven't noticed them before, thought I've visited Duncanamp pages many times. Thanks man !

how did you come up with a 1,7K PrimImp? What primImp would you suggest using with 2 EL34's that will produce 80W out? A VOX AC50 has 3,5K, and gives 50w. (biased at 22V (~35-45mA per tube))
Maybe around 2,5-3K with -37V (~470V on plate)? (voltages taken from the schematic.)

I'dont really need the 120W you see, and I want it to sound as close to the original as possible!!

It is very frustrating with all the schematics out there, and only a few state the Primary Impedance.