Single ended ODS

[Colossal]

Hi Colossal
The separate 1MA Masters seem to give a good balance between the channels, the OD side is still louder of course but OD gain on 5 and OD Master on 5 gives a slight boost compared to the clean gain on 5 and Clean Master on 7.One thing I have noticed is that the EL84 seems to be slammed too hard with the Masters on full giving a harsh edge to the distortion. I've tried a 220k resisitor to ground before the grid stopper on the EL84 but to no effect. Any suggestions?

Cheers Andy

Andy,

Thanks for getting back to me. These ODS SE amps have piqued my interest for quite a while starting back with Mat's HRM SE. All of the variants seem to really sound quite good. I keep hearing a common quality to them which sounds remarkable.

I like the idea of separate channel masters but I was thinking that 1M (although still within the limits of the tube specification) would push the tube way too hard. I was really wondering about that. In fact I was thinking about 250kA to ground as each master. You might also try 220k as a fixed resistor in front of each 250kA master. What about some additional padding for the OD channel? I also recall Mat using 280k (for the grid resistor) on his EL84 HRM SE.

Have you tried any other primaries than 5k? Also, what plate voltage are you running your EL84 at.

I am very interested in one of these for both effected cleans and slightly delayed, soaring OD (with more gain but without getting too hard and boorish sounding) for a fusion/progressive sound (e.g. Holdsworth). I keep hearing Holdsworth in that thick midrange Dumble honk and although most guys seem to use the more "polite" aspect of the OD, I'd like to open it up a bit more gain-wise but keep it super smooth.

Thanks,
Dave

[maxfac]

Hi Colossal
I've tried Mat's 280k grid stopper to the EL84 and it seemed to take off the high frequencies a bit but not much effect otherwise.
I'll try swapping out the masters for 250k pots and report back. What would the extra 220k resistor in front of the pots do ?
As for the voltages on the EL84, I've got one of Dana Hall's VVR boards installed, it's configured to just effect the EL84 plate and screen so I can set them from 353v/347v down to 35v/34v. Preamp voltages are about where they should be for high plate ODS.

Thanks for your interest - Cheers Andy

[Colossal]

Hi Colossal
I've tried Mat's 280k grid stopper to the EL84 and it seemed to take off the high frequencies a bit but not much effect otherwise.
I'll try swapping out the masters for 250k pots and report back. What would the extra 220k resistor in front of the pots do ?
As for the voltages on the EL84, I've got one of Dana Hall's VVR boards installed, it's configured to just effect the EL84 plate and screen so I can set them from 353v/347v down to 35v/34v. Preamp voltages are about where they should be for high plate ODS.

Thanks for your interest - Cheers Andy

Andy, I was wondering about your voltages and bias point because in the two other examples of SE ODS type amps we've seen (which are not reporting overloading the output tube), those builds are based on center-biased Class A power amp conditions, pretty much right out of the tube manuals. We are seeing 5k primary which should give the most headroom for Class A conditions at roughly 250VDC on the plate. I'm not saying there's anything wrong with your amp in comparison, just noting the differences. I'm thinking about this and asking because I'm thinking about building one of these myself. It's interesting that you note that your preamp voltages are where they should be for a high-plate ODS and I know that's very important to getting the sound right. You can set up a preamp with Dumble values, but the operating point of the gain stages is pretty important!

I was thinking of 250kA as a master volume/grid leak as this works very well for an EL84, even lower and there's plenty of drive. The classic Marshall 1974 circuit (2xEL84) uses 470k/470k and those amps are all about output tube distortion. In a 4xEL84, four gain stage push-pull amp I have, the post phase inverter master (which essentially what your channel masters are, variable grid leaks) I have it set for a max of 160k and there is still plenty of output tube drive.

With the extra 220k resistor forming a voltage divider with a 250kA master, that will form the Thevenin equivalent of 220k || 250k to AC which is 117k so this should load down the previous stage a bit more by changing the output impedance. DC is blocked from the preamp so bias is being set with the grid leak/master as usual. I was wondering what value you used for your cathode bias resistor on the EL84. Your amp has to be closer to cutoff (lower current/higher plate voltage) at idle than the other guys running 250-260V and 130R for a center bias point and max output of about 11W at idle.

With the Dumble design, the "magic" is in the preamp so I'd think the power section would be pretty much for a clean reproduction of the incoming signal (with some added harmonic content). Although, I do believe I remember reading that Mat (was it Mat? ) said he was getting some power tube distortion too.

[maxfac]

Hi Colossal
Thanks for your efforts to help me with this. I've put the 250kA Masters in this afternoon and it's made such a positive difference I'll try adding the other 220k resistors when I can figure out where to put them. The amp has less volume though, it was very loud before, quite a bit louder at the same settings than the Marshall Class 5 and Laney Lionheart 5 watt I've pitted it against. FWIW I'm using it with a homemade 1 x12 open back cab loaded with a early 70's Celestion G12 20 watt speaker.
The cathode bias resistor is 330 ohms which to be honest I've always thought was a lot higher than most were using, but I've done the maths several times and it always comes out at around 11 watts disappattion. I've tried using readings at various VVR settings and it always comes out about the same. I'll send you my readings if you'd like to check them, maths was never my strong point!!
I've only used a 5k OT, the amp is a recycled Blackheart BH5, maybe I'm getting some high frequency artefacts from the OT it is quite small.
I'll amend my schematic to reflect the changes I've made and repost so you can see what I've done, always better to learn from someone else mistakes before you go and make your own !!!

Cheers (and thanks again for your input) Andy

[Colossal]

Hi Colossal
Thanks for your efforts to help me with this. I've put the 250kA Masters in this afternoon and it's made such a positive difference I'll try adding the other 220k resistors when I can figure out where to put them.

That's great to hear that reducing the master helped. Apart from the volume drop, can you say that the tone is where you want it? You might not need to go any further if you feel the ugly distortion is dealt with for the conditions where you intend to play the amp.

The amp has less volume though, it was very loud before, quite a bit louder at the same settings than the Marshall Class 5 and Laney Lionheart 5 watt I've pitted it against. FWIW I'm using it with a homemade 1 x12 open back cab loaded with a early 70's Celestion G12 20 watt speaker.

I expect there would be some volume loss, but hopefully with it, the ugly distortion products from pushing the tube to hard. You might consider offsetting the volume loss with a more efficient speaker. I have a single EV 12ML Classic (8R) in a wide body, rear ported cab, and honestly, and I'm very sad to say this, it just blows my closed back 2x12 with some Greenback reissues. I love that cab and speakers and have always been a bit of a Celestion man but for the music I am playing, the EV has incredible, tight low end, is ighly articulate without excessive detail and it is loud. It is louder and more full sounding than the pair of 12" 25W Celestions. So this might be something to consider in shaping your tone. I have no doubt that these SE amps do sound great, we've heard that with Honeydip's and Mat's amps. It doesn't hurt that they are great players as well!

The cathode bias resistor is 330 ohms which to be honest I've always thought was a lot higher than most were using, but I've done the maths several times and it always comes out at around 11 watts disappattion. I've tried using readings at various VVR settings and it always comes out about the same. I'll send you my readings if you'd like to check them, maths was never my strong point!!

The cathode resistor is high, but your plate voltages are much higher than the other guys who are running their amps more center biased at low voltages. I haven't done the math, but your value sounds right. My 4xEL84 metal amp is cathode biased and I am sure my voltages are right where yours are. Crushing sounding amp. That is the amp I'm running the grid leaks at 160k wide open and it sounds really good. It just kills paired with that EV.

I've only used a 5k OT, the amp is a recycled Blackheart BH5, maybe I'm getting some high frequency artefacts from the OT it is quite small.
I'll amend my schematic to reflect the changes I've made and repost so you can see what I've done, always better to learn from someone else mistakes before you go and make your own !!!

I really appreciate you trying these changes and reporting back as it will certainly help others including me who might build one (or two!). In my opinion, I would say you could do very well to upgrade that OT if you like the amp enough to keep. Those generic OTs sound weak with anemic low end and fatiguing highs. The transformers are the foundation of the amp's tone no matter how small.

I will do a load line for your conditions and for an EL84 amp running 250V at 5k and repost. I am hoping to get to it today but may have to push it till next weekend as I will traveling this week. I've very interested to see how your future mods go.

Also, I noticed you did one of 10thTX's mods lowering OD2 cathode bypass cap to 1uF. I think that is a very good idea. It will really help tighten up the low end instead of pushing mush in to the power amp. I have a four gain stage amp that has 0.68uF straight across three of the four cathodes in the preamp and there is definitely a subsonic harmonic being generated that is absolutely huge. The amp's low end is super punchy and sounds like a bass module for a home theater. Really quite surprising. So it does not take large amounts of bypass capacitance to make a very fat sounding (high gain) amp.

Cheers (and thanks again for your input) Andy

Thank you for posting your results!

[wicker]

Do you have any clips ? And what guitar do you have ? I'm interested in nice SE (6L6) build.

[maxfac]

Hi Wicker
No clips as yet, hoping to get around to it soon. I've got a number of guitars (Tele, Strat, LP P90' s and HB, ES335 ) I can get my hands on if I ask nicely, so I'll can post a good variety, anything you'd particularly like to hear ? Cleans and/or OD? I can't say much for the standard of my playing though!!!

Regards Andy

[maxfac]

Hi Colossal
Thanks for your suggestions, I added the 220k's on the channel switch before the Master Vols and it's got rid of all the harsh distortion I was getting, balance between the clean and OD channel is easily achieved though the OD channel is louder for the same master volume setting but there are two extra gain stages so I suppose no real suprise there.
I've also changed the cathode bypass cap on CL1 to 1uF to match CL2 as the cleans sounded a bit dark,suprisingly this doesn't seem to have affected the OD channel voicing much at all. I've also upped the treble bypass caps on the Master vols to preserve the sparkle as the masters are lowered. I've posted a revised sch. file and gif. to reflect the latest changes.

Regards Andy

[maxfac]

Oops forgot to mention I also added some NFB to OD2 which also helped to tighten the bottom end up as well.

Andy

[Colossal]

Andy,

Excellent to hear that you were successful in ridding the amp of the problems you were having! As for lowering the bypass caps, I think that's a good idea and I like the use of small bypass (film) caps and small coupling caps where possible, gradually increasing the low end content later in the amp.

So how are you feeling about the tone now? Interesting use of NFB! For your bright caps on the masters, did you try a few different values to find the right amount of sparkle for each channel or just up them based on your gut feeling and what was in your parts drawers?

Cheers,
Dave

[maxfac]

Hi Colossal
The tone is very good now (IMHO) I'm especially liking it with my Strat it seems in OD to have just enough of that compressed quality that I hear in the Dumblesque clips others have posted. With my P90 loaded Les Paul smoothness goes a bit but backing off the vol on the guitar gets seems to dial it out.The NFB idea think I got from Merlin Blencowe's ( Amp Wizard) Book and from Sewatt ? It really made quite a difference, sort of reined in a bit of the aggressive quality which I did'nt want.
The bright cap values were arrived at by listening carefully as the masters were lowered and taking an guess at which values might give me what I wanted, I got it right first go,unusually for me!!
Have you had chance to check my maths on the cathode resistor values?all voltage are on the schematic I posted.No worries if not just curious.

Regards Andy

[maxfac]

Hi
Just thought I would post an update. I decided to go back to a single master volume, I missed the tone shaping capabilities of the OD level (Ratio). I changed the mid cap values to 0.0022uf and 390pf which gave a more useful boost. I also added a passive series fx loop which worked as long as the output level on the fx unit (Pod HD500) was kept really low, if turned up the amp howled like a banshee (not a lot of use) after a lot of rewiring and head scratching I finally tracked down the problem to the fact that when the fx loop was in use the nfb loop was interrupted by the Pod, I could have just diconnected the nfb but I like the tone better with it. I got around this dilemma (thanks to Valve Wizard's (Merlin B) book) by using a switched stereo return jack to disconnect the nfb when fx loop is in use,when not in use the nfb loop is reconnected. He recommends using this method to get a bit more gain to drive a passive fx loop a little better.I've included an updated sch. and gif file.
Still no clips, sorry but I'm working on it.
Cheers Maxfac

[Bob Simpson]

What is the wattage rating of the !MA pot in the VVR circuit?
Is a standard 1/2 watt enough?

Thanks!

Bob Simpson

[maxfac]

Hi Bob
It's one of Dana Hall's VVR boards, cathode bias up to 50watts output, it's a 0.5 watt pot, I've never had a problem with one.

Regards Maxfac

[hywelg]

Andy, fancy bringing it into Dave Buckleys studio and recording some clips there? I for one would love to hear it.

I'm still working on him to have another amp demo day.

Hywel Harris