Significant redesign of 5879&ECL84 amp - request help finding schematic issues

[boodle]

All,
I was on the hunt to build Sluckey's Dual Lite using 1-5879 and a quad of ECL84 tubes.  DummyLoad pointed out the possibility of instability from using the triodes from the ECL84 tubes for the 18W preamp portion of the amp.  That seemed like wisdom to me.

Thread here if interested:
https://el34world.com/Forum/index.php?topic=30268.msg333417#msg333417

Due to that feedback, information from Sluckey that he usually just uses the Vox side of the Dual Lite, and the fact that I HAVE an 18W that I built and like very much, I decided to redesign the amp with just the Vox side and use the 2 unused triodes help add tremolo and reverb.

I have used the power tube grid wiggle tremolo design from a Matchless Hurricane.  One triode, no new power supply needs.

For the reverb, I wanted to use the other triode for the recovery stage to limit how much gain it could add into the signal in case of any internal feedback, but still utilize the triode.  I found the MOSFET reverb driver circuit from another thread here.  I WILL need to add a reverb transformer, but I think it will likely help the reverb sound by rolling off some top and bottom that the MOSFET wouldn't limit.

What I am hoping to gain from your expertise is a heads-up to any issues you see that I could make for myself in my current schematic.

Any help is very welcome!

PS: If the triode in the reverb recovery circuit is an issue, I plan on replacing it with a MOSFET and call it a day.  I don't have any great preference for specific reverb sounds.  Just would like to add a little "space".

[tubenit]

A couple of thoughts.  I've built both ECL84 and 6BM8 amps.  Four ECL84 tubes ought to be pretty close to the two 6BM8 tubes. The 6BM8 tubes are still being manufactured and they sound great, IMO.  A pair of ECL84's were around 3-4 watts, IIRC?  6BM8's were around 7-9 watts I think.

You could consider a mosfet CF and then use the 2nd triode for a psuedo active FX loop.  I've done this with two of my current amps and have been pleased with the results. Using that approach, you can then use reverb or delay pedals in the FX loop.


Your design uses a 5879 & 4 ECL84 tubes for a total of 5 tubes.  The Minimalist uses a 5879, 12A_7 and 2 6BM8 tubes for 4 tubes total.

Take a look at the threads below for more ideas:

(8) SoLow Watt ECL84 amp - The Amp Garage


Bumble B+/Solow Watt ECL84 (el34world.com)


HoSo56 minimalist & 6BM8 tubes (el34world.com)


sound clips of the 6BM8  Minimalist :   
https://soundclick.com/share.cfm?id=13741608
https://soundclick.com/share.cfm?id=13741593


This is another 6BM8 push/pull amp very similar to The Minimalist but uses a 12AY7 instead of a 5879 in V1.
https://soundclick.com/share.cfm?id=14315281


With respect, Tubenit

[boodle]

Tubenit,
Thank you for your response and thoughts.  Well considered and helpful as always.  I appreciate it.

I will think that through.  Thus far, I have done some chassis work and planned a layout and board, so most of my time thus far is mental work.

One thing that has me concerned about the 6BM8 is that the only new ones that I see are quite pricey.  The NOS ones are less cost than the new ones.  Have any tips on finding them on the cheaper side? 
[There is a Russian site (Tube-Store.com) that has Russian military equivalent 6BM8 tubes for ~$10, but they also currently have the ECL84 for $0!  Makes me a bit nervous that that I could put 100 ECL84's in the cart for a total cost of $0. 
Actually, just recheck the site and the 6BM8's are now free as well.  Something is broken there.]

Would still love any feedback from anyone on the schematic, even if it just turn out to be an academic exercise.

Thanks,
Tim

[boodle]

Tubenit,
One other question:  Do you have any thoughts on the tone difference between the 6BM8 and the ECL84?
Tim

[PRR]

... ... ...they also currently have the ECL84 for $0!  .....I could put 100 ECL84's in the cart for a total cost of $0. ....the 6BM8's are now free as well.  Something is broken there.

"'Jeff' made a very simple error – the kind of error any one of us could have made.
The boss ...asked Jeff to change the listings for every product on every store.
Jeff had inadvertently zeroed the price column on every product. Hundreds of them.
Spreadsheets are hard, right?"
https://www.theregister.com/2023/08/14/who_me/

[boodle]

   I will be requesting Jeff to sell me some tubes!

[tubenit]

Do you have any thoughts on the tone difference between the 6BM8 and the ECL84?

I've not had the ECL84 amp for years so it's hard to remember exactly what it sounded like?  I will say that I am a big fan of the 6BM8 tubes which I think have a reasonably warm and smooth tone to them. 


Keep in mind that with these pentode/triode tubes that it's like getting a dual triode and two pentodes at the price of two tubes.

IIRC, I think the ECL84 tubes were fairly easy to overdrive and didn't have much clean headroom?


With respect, Tubenit

[boodle]

Thanks, Tubenit!

[boodle]

My "Tiny Vox" is built and sound quite nice.  I will post audio files when my tweaking is done.

It is housed in a Traynor TS-15 cab and chassis.  I found an empty one online.

I am using WGS's ceramic 8" and their alnico 8".  Both "American" toned speakers.  I have a Weber Blue Pup8 on order to try it with the other 2 8's.

The amp is plenty loud to be a practice and gig amp if a PA is used.  To that end, I want to bias the ECL82 PP pair to provide maximum headroom.  I have tried 2 different common cathode resistors for the PP pair to date: 
430R - borrowed from one of Tubenit's ECL82 designs
215R - closest I have to what a Philips datasheet states for a Va=250V.

Below are 2 versions of the schematic, same doc title except with 430 or 215 at the end.  [Not that it matters for this discussion, but I currently have pulled the bright switch though still shown.]

Here is a summary of idle voltages:
                          430R                   215R
Plate(pin 6)         254,256              243,245
Screen (pin 7)     246,250              233,236
Cathode (pin 2)   20.5                    17

Voltages shown are V2,V3 Pentodes.  I did not attempt to match tubes.

I have also attached what I think are the pertinent pages from the ECL82 datasheet.

My question:  Using the data above and datasheet, can someone direct me as to what cathode resistor to use to get max headroom before significant power tube distortion?

[PRR]

Bias and max power are opposite ends of the spectrum.

In a car, what idle speed makes it go fastest? Doesn't work that way.

Yes, in *cathode resistor* bias, running cool leads to a glitch near max output. My inclination is to run 99%-110% of rated dissipation at idle. However I am not sure this glitch is anti-musical for guitar.

Anyway: if you want "max", why use ECL82? EL84 is brawnier and readily available on a Saturday night.

[boodle]

PRR,
Wanted to use an oddball tube in a design, so purely experimental.  Turned out to be great.

Based on your response, I am assuming that my idle voltage measurements are not of much use in determining a cathode resistor for maximum headroom?

Datasheet says 7W max for audio output for a single pentode.  So, 14W for P-P pair at idle?  Data sheet lists 10.5W for Va=250V with 220 ohm cathode.

Calculate power from measurements by Vak*I, where Vak is plate to cathode voltage and I is current calculated using Vk and Rk?

[PRR]

> Datasheet says 7W max for audio output for a single pentode. 

??? I see 4 watts dissipation per tube, which means less than 2 Watts sine audio per tube.
https://frank.pocnet.net/sheets/030/e/ECL84.pdf

[boodle]

PRR,
Thanks for your replies.

I am using ECL82's on this amp.  The amp was originally an ECL84 build, but that changed after input from Tubenit.

Sorry if I was unclear in my recent post.

[PRR]

A pair of ECL82 is good for 10 Watts.
http://www.r-type.org/pdfs/ecl82.pdf