Single ended EL34 amps

[Jay Menon]

Single ended versus push-pull amplifiers. Do they feel and sound different?

Lately I've been enjoying the traditional British Marshall guitar amp sound (EL34 based push-pull amplifier).

I now have a couple of low watt push-pull amplifiers (based on 12AU7 and 12BH7 output valves).

My fascination with the EL34 however, remains - though admittedly I understand very little of how guitar amplifiers work.

The question is however - do fairly expensive single-ended Class A EL34 based amps (eg. Cornford Carrera, Cornell Plexi 7, Fargen Mini Plex) sound and feel like the quintessential 'British Stack'?

[ef37a]

Morning Jay.
The single ended EL34 amplifier is a design SNAFU as far as I am concerned!

First off, SE designs must be biased to class A* and in the case of the 34 this means an anode current of 100mA and for a poultry 11 W output an anode dissipation of 25W (250V HT) Add the massive heater current and the total power consumption of the valve is some 38watts.

Then, an SE amp must have a much bigger OPTransformer than a push pull one of comparable power because the unidirectional DC in the winding magnetizes the core and causes distortion (especially at LF) . To obviate this a an air gap of about .2mm or so is made in the core (which has to be "stacked" Es and Is i.e. cannot be interleaved) this gap greatly reduces the efficiency of the transformer so it must be bigger, more expensive and runs hotter for any given power output.

Because SE amp OPStages are not "balanced" as are PP amps, there is no power supply rejection and thus they are harder to keep quiet, hum. And we spoke of "sag"? Not going to happen to a basic SE amp to any degree.

Contrast this with a push pull design using the venerable but delightful 6V6? Ok, you need two OP valves but they are much smaller and cheaper. Yes you need a PI but it will give about 30dB of gain. There will be savings on the OPT and mains traff since the whole OP stage would only consume some 15-20W (fixed biased, bit more for cathode bias but see later) and so the PSU can be much smaller. You need some nice big caps for transients but decent electrolytics are cheap these days. And for this small additional complexity you get 15W easily and up to 20W at max thrash.

"Later": Cathode biased amps limit very quickly. Under drive the cathode voltage rises and tends to bias off the valves. This produces a certain "tone" beloved by some and the most famous CB amp is of course the Vox AC30. Big, snorting stadium strippers are almost always fixed biased class AB and again, the function of the OPstage (no limiting) gives them a specific sound (and "feel" some would say!) .

Blackstar have never and I am sure probably WILL never produce a single ended guitar amplifier for the technical reasons I have outlined. (6V6? Yes, never been a fan of the EL84 but you would get a similar amount of welly. Different sound tho'but) . The PP design of the Five was an attempt to get a "big amp" sound at low wattge. Whether they succeeded is for others to say but they had a better chance with PP than if they had gone the "easy" EL84 route!

*Class A: No valved guitar amplifier stays in class A mode when driven hard (and virtually no valved hi fi amp either!) . It is a term I should like to see banished from the forum pages along with "rms" watts!

Dave.

[Folderol]

But... but... what about all that lovely even-order 'warmth' you get from single ended O/P

Sorry, Dave. Couldn't resist

[John Egan]

I've always thought that the type of valve is secondary to the way the designer voices the amp. My experience of push-pull amps suggests that they are very different from single ended, but this might be because the single ended ones I've played are much smaller and this inevitably has an effect on the player's perception. I've never tried one of the bigger Laney amps which apparently use multiple single ended tubes, but I am curious as to how they sound.
As far as valves are concerned I tend to prefer 6L6 and 6V6 to EL34. I used to have an old (1969) Marshall Superlead, but I hated it. Interestingly, I still have a Selmer Treble and Bass 50 Mkii which uses EL34s and is lovely. No accounting for taste, I suppose.

[ef37a]

I agree John, how the various stages of the pre amp sections overload and at what frequency and order has a much bigger effect on the sound of an amp than the choice of OP valve IMHO. (of course, changing 34 for L6 in any GIVEN amp circuit will probably make some tonal difference but not, I would aver as much as the purveyors of NOS and boooteek bottles would have us believe.)

I would also doubt that there is enough difference in the sound of amplifiers cooking away at 40,50 100watts all the time compared to more efficient designs to justify the cost and "un-greenness" of the designs?

I urge everyone interested in guitar amplifier design and sound to get Merlin Blencowe's book on "Tube Pre amps* . In it he gives dozens of examples of ideas to get a variety of tones from pres that, to my knowledge have never been seen in commercial amps? Just the things you can do with an EF86 would keep you mackling till Christmas!

Would that I were 20 years younger and had some energy and two good eyes!

*Worry not! I think the publishers must have put "toob" on the cover. The book quite rightly stays THIS side of The Pond and speaks only of valves (unlike SoS!)

Dave.

[John Egan]

Thanks for the recommendation, Dave. That's not a book I've come across, but I'll see if I can find a copy. I find that the Dave Hunter books on amps are a good read.
As a matter of interest, the best amp I ever played was at the Rockpalast in Kiel in the mid 1960s. Unusually for that era, the club provided PA and backline (it gave them control over volume).
The backline was a row of Fender Dual Showman amps in blonde tolex - each piggy backed on a 2x15" cab. It was the first Fender amp I'd ever played through and it was wonderful. I remember wishing I could afford one to replace My old AC30, but the price would have been astronomical! That's when I bought my Selmer Treble and Bass 50 - the closest I could get to the tone that I could afford (just)!

[Martin Walker]

I still have a Selmer Treble and Bass 50 Mkii which uses EL34s and is lovely.

Wow, that product is (as I believe these modern DJ chappies term it) a 'rave from the grave' John

Are we talking about this beastie?



Martin

[Jay Menon]

That Selmer looks a lot like a Dumble...

[ef37a]

I have found a schematic for that Selmer T&B Mkll.

It is fixed biased and therefore class AB and runs the EL34s at a modest 400V. Note, the bias really is fixed! No adjustment pot and so if it ever needs modern valves they could be quite a way off the original Ia point. I have had to mod Artisan 100 bias circuits slightly to accommodate modern, poor QC valves.

Unless the amps are of great collector value I would also fit a 1 Ohm 10W bias sense resistor in the cathodes.

Dave.

[John Egan]

I still have a Selmer Treble and Bass 50 Mkii which uses EL34s and is lovely.

Wow, that product is (as I believe these modern DJ chappies term it) a 'rave from the grave' John

Are we talking about this beastie?



Martin

Yes Martin,that's the one. I bought mine new in 1965. It cost me £50. It replaced my old AC30Super Twin. I used it with a 4x12" cab with two Goodmans Audiom 61(?) and two Richard Allen 12s with samarium cobalt magnets. Lovely sound (lousy technique!).

[John Egan]

I have found a schematic for that Selmer T&B Mkll.

It is fixed biased and therefore class AB and runs the EL34s at a modest 400V. Note, the bias really is fixed! No adjustment pot and so if it ever needs modern valves they could be quite a way off the original Ia point. I have had to mod Artisan 100 bias circuits slightly to accommodate modern, poor QC valves.

Unless the amps are of great collector value I would also fit a 1 Ohm 10W bias sense resistor in the cathodes.

Dave.

Dave, would that mean I'd need to undo the screws holding the back panel?

I have actually fitted a pair of JJ EL34s and they sound pretty good to me, though it's used at fairly modest volumes these days. I don't gig any more, I just enjoy writing songs and recording.

I do have quite a few amps, including a number of old ones. And too many guitars for my own good.

[John Egan]

That Selmer looks a lot like a Dumble...

I thought that was Link Wray record, Jay.

Seriously, I've read about Dumbles but I've never seen one. One of our local amp techs, Jeff Pugh, is the man behind Volt amps and he makes Dumble style amps.

Jeff made me a J45 style amp and its the best made amp I've ever seen. Also, it sounds great!

[Folderol]

It does indeed!
You would also need to lift the wire off pin 8 of the EL34 valve bases and insert 1 ohm wirewound resistors as Dave specified.

This enables you to safely and accurately measure the total valve current. 30mV across this resistor = 30mA. Even so, it's not for the faint hearted.

[John Egan]

It does indeed!
You would also need to lift the wire off pin 8 of the EL34 valve bases and insert 1 ohm wirewound resistors as Dave specified.

This enables you to safely and accurately measure the total valve current. 30mV across this resistor = 30mA. Even so, it's not for the faint hearted.

I'm definitely in the faint hearted camp when it comes to that kind of adjustment. When I turn the volume control up, the excitement can make me go temporarily blind!

[Jay Menon]

Hi John, Geoff Pugh is a great guy...

He made me a HRM version of a Dumble style amp - With too many features to list, yet each one incredibly relevant and useful. That's the best amp I own too.

He even incorporated a custom faceplate 'the JM100' which for me was the absolute icing on the cake ...

[ef37a]

It does indeed!
You would also need to lift the wire off pin 8 of the EL34 valve bases and insert 1 ohm wirewound resistors as Dave specified.

This enables you to safely and accurately measure the total valve current. 30mV across this resistor = 30mA. Even so, it's not for the faint hearted.

I'm definitely in the faint hearted camp when it comes to that kind of adjustment. When I turn the volume control up, the excitement can make me go temporarily blind!

Sorry! Did not know you were non technical. It would be a good idea though to get a tech to look at the amp. Even if you don't want the circuit touched he can check the bias by the (even more hazardous!) transformer resistance method. Running the valves unnecessarily hot will do the transformers and the GZ34 no good at all. 30mA is a hot as I would want to run them and all "our" FB amps run at 25mA (except the HT-5) .

Dave.

[John Egan]

It does indeed!
You would also need to lift the wire off pin 8 of the EL34 valve bases and insert 1 ohm wirewound resistors as Dave specified.

This enables you to safely and accurately measure the total valve current. 30mV across this resistor = 30mA. Even so, it's not for the faint hearted.

I'm definitely in the faint hearted camp when it comes to that kind of adjustment. When I turn the volume control up, the excitement can make me go temporarily blind!

Sorry! Did not know you were non technical. It would be a good idea though to get a tech to look at the amp. Even if you don't want the circuit touched he can check the bias by the (even more hazardous!) transformer resistance method. Running the valves unnecessarily hot will do the transformers and the GZ34 no good at all. 30mA is a hot as I would want to run them and all "our" FB amps run at 25mA (except the HT-5) .

Dave.

I love to read about amp design, but I always thought that before poking about the innards of my amps with a blunt screwdriver, I would need a lot of tuition. I did build one of Jeff Pugh's 5-watt amp kits, but then I saw the amp that he made for me and decided that my efforts were laughable by comparison. I'm sure that modding amps is best left to the experts. I'm OK with wiring guitars, though.
Thanks Dave and Folderol for the helpful comments. I will get a tech to check the amp out - and maybe some of the others I've got - Laney Klipp 100; Simms Watts 100; Roost 100; Sound City 120; Kelly 50 2x12" Combo; '73 Fender Dual Showman Reverb; '76 Twin Reverb, among others. I like amps.
By the way Hugh, I don't think the knobs on that Selmer are the originals. The originals on mine had a habit of falling off. so I had to replace them. They were definitely not chickenheads though.

[John Egan]

Sorry Martin, the above comment on the Selmer chickenhead knobs was meant for you. I had been reading some of Hugh's posts on a different topic just before I posted and had a senior (senile) moment.

[Martin Walker]

No worries John - we all have senior moments from time to time (some more than others )

As it happens, I've never been a great fan of chicken head knobs. I can see their obvious functional and visual advantages, but I can't bring myself to warm to their aesthetics

Martin

[Jay Menon]

Also called a 'C-R-A-F-T' moment. You legally licensed to have them once you hit 50 ...

[John Egan]

Thanks,Martin. Thanks Jay.
I think I remember 50..........I think I do.................maybe.................

[John Egan]

Single ended versus push-pull amplifiers. Do they feel and sound different?

Lately I've been enjoying the traditional British Marshall guitar amp sound (EL34 based push-pull amplifier).

I now have a couple of low watt push-pull amplifiers (based on 12AU7 and 12BH7 output valves).

My fascination with the EL34 however, remains - though admittedly I understand very little of how guitar amplifiers work.

The question is however - do fairly expensive single-ended Class A EL34 based amps (eg. Cornford Carrera, Cornell Plexi 7, Fargen Mini Plex) sound and feel like the quintessential 'British Stack'?

Jay, having rudely hijacked your thread, I thought I'd better post something relevant for a change.
I remember a couple of years ago Rick - late of Sounds Great Music near Wilmslow - demoed me a Laney Cub 12R, which is a single ended EL84 amp, and he absolutely nailed the classic Marshall tonality at low volume. Very impressive even if it's not my thing. I don''t know if you've ever tried one?

[Jay Menon]

No problem John - hijacking is good, especially when it gets the big boys arguing, and we can sit back and watch the fun

Re: the British / Marshall sound, my Blackstar HT5 has totally nailed it - and that too with all the knobs at 12 o'clock (except the US/UK dial of course). And I really couldn't care less if it's 'all valve' or not.

The Dumble HRM style amp that Geoff Pugh made is phenomenal too - and since it is a largely preamp based tone, sounds great at low volumes.

Here's another nice one - a hand-wired affair by Andy Dokken of Rat Amps in Cornwall based on a 12AU7 in push-pull configuration - with powerscaling.

It's nice to be an amp geek...

[ef37a]

No problem John - hijacking is good, especially when it gets the big boys arguing, and we can sit back and watch the fun

Re: the British / Marshall sound, my Blackstar HT5 has totally nailed it - and that too with all the knobs at 12 o'clock (except the US/UK dial of course). And I really couldn't care less if it's 'all valve' or not.

The Dumble HRM style amp that Geoff Pugh made is phenomenal too - and since it is a largely preamp based tone, sounds great at low volumes.

Here's another nice one - a hand-wired affair by Andy Dokken of Rat Amps in Cornwall based on a 12AU7 in push-pull configuration - with powerscaling.

It's nice to be an amp geek...

I know I should not "knock the competition" and I don't as a rule but!
Rat's nest indeed! FHS why couldn't they run those screened wire UNDER the turret board?

Dave.

[Music Wolf]

Jay

Since you're having fun experimenting with different amp configurations, and since it would also seem that you are working at low volume, have you considered a Kemper profiling amp?

I've got one and I absolutely love it to death. No, it hasn't got any glowing glassware and it may or may not have the true feel of the original amps - but it is capable of producing excellent results and, most importantly, it's allows you to experiment (you can also purchase lots of additional profiles at pocket money prices).

I've come to the conclusion that the 'Vox' sound is my thing but I'm also loving the Two Rock profiles. At the weekend I was having great fun with the JC120 for clean / chorus sounds.

Now here's the really good bit, at least if you are into recording - re-amping with two amps. I've had the Kemper for a while but I've only just started to re-amp. Right from the start I wired it to my AI so that I could easily record the profiled sound and the DI at the same time (which I've been doing for bass) but over the weekend I did this for guitar, initially so that I could comp a solo then re-amp it as a single take (otherwise known as cheating). As it was I got the track down in a single take but it did allow me to re-amp with two completely different profiles (using two amps is something that Andy / Zen of this parish first brought to my attention). Whilst I was playing I was getting the feel of one amp + effects but during re-amping I stripped the effects off the Kemper and applied them in the DAW for extra control.