50 watt EL34 build questions

[pdf64]

I tried everything from 20K all the way up to 370K - same sound, same behavior, same affect - super clean and reduced volume. .. You can see in the gut shots posted that the tail resistor is grounded along with the .1uf cap coming off of pin 7 of the PI - this is the NON NEGATIVE FEEDBACK configuration. In my experimenting with the NFB resistor I lifted the tail resistor and cap from ground and connected them directly to whichever value resistor I was testing, and from there the 'supposed' NFB resistor was connected directly to the speaker jack terminal which is tied to the 8 ohm tap on the OT secondary. What the hell am I doing wrong?

It seems that the ground return for the LTP tail resistor is via the feedback resistor and OT secondary.
That squares up with the observation that changing the feedback resistor value doesn't seem to do much.
I suggest that you add a 4k7 resistor between the 15k LTP tail and ground.
That's the feedback node for the feedback resistor and 0.1uF.

[Jana]

You say you have your DMM set to ma. Are you measuring across a resistor or are you breaking the circuit someplace and inserting your DMM? Most likely a bias probe has a resistor in it and you are supposed to measure the VOLTAGE across that resistor so you can calculate the current.

[WRC34]

Was a little busy during the holiday but I had time for some brief changes & testing.

Read bias with mV setting and it stabilized after a little while. I think I was taking readings before the amp had a chance to calm down after being turned on, plus it was right after I'd built it and powered up the first time (I don't have a variac for gentle awakening). Maybe everything has kind of settled into place now?

Installed a 5.6K resistor between the tail resistor and ground per suggestion of pdf64 (thanks!) and boom! sounds great!! In fact, it sounds tamer than my JTM45 clone with stock NFB, which has me scratching my head in a different way than I was at the beginning of this thread, but now with a smile on my face since the amp sounds excellent. It sounds how I wanted/intended it to sound: nice & loud, breaks up good when turned up but retains decent note/string clarity at higher volumes. The last 1/4 or so of the volume kind of turns to mush, but the amp is at full volume well before the dial gets there so hey, what the heck. However what I still don't understand is how to wire up NFB without a presence pot.

I'm leaving this one as it is for now since I'm excited to move on to my next build There is some unfinished business here but I'm not bothered by it too much.

Thanks again everyone & have a great New Year

[WRC34]

A little after the fact here but I wanted to add this nonetheless:

I'm not too great at drawing schematics, so here is a layout of the amp exactly as it is. I got the chance to use it "full blast" at rehearsal a week or so ago and it sounded much better than I expected. Very quiet at idle, and roaring at full volume. It is very clear with note definition reminding me of a Twin Reverb and breakup like a JTM50 (which makes sense because the power section is lifter from it). It was very civil despite the lack of negative feedback, not loose sounding in any way, which surprised me. I ran the amp full out for almost 3 hours without a hiccup. The cab was a 2x12 with one Celestion Gold and one Heritage G12H 55Hz.

[bruce egnater]

You mentioned earlier you had a bias tool set to milliamps. What gadget is this? The reason I ask is because they typically measure the VOLTAGE across a resistor as opposed to measuring current which must be done in series.

[WRC34]

Thanks Bruce (and Jana!) I decided to quit messing around and install the 1 Ohm resistors between pin 1 & ground on the EL34s. Using mV I was able to get the bias in perfect* range. Funnily enough, it seems I prefer them biased on the colder side despite wanting to use the amp for aggressive rock tones 85% of the time. To me there's nothing 'cold' about the sound but I do think that helps with the clarity and 'string separation' for lack of a better way to describe it.

*to my ears

[Structo]

EL34 pins 1 & 8 are connected, correct?

[WRC34]

Hi Structo

Yes, pins 1& 8 are connected on the EL34s. Plate voltage is around 460VDC, and I've got the EL34s biased around 32-25mA (one tube draws a little more than the other). To me this sounds really good.

[WRC34]

I finally got a chance to do a brief demo of this amp tonight. It's a little goofy and has the amateur 'my amp is too loud for my camera' thing going on, but here it is nonetheless.

https://www.youtube.com/watch?v=d5l9t0brBDw

Fingers crossed I'll get around to making more demos of some other amps soon

[Garthhog]

That is a nice sounding amp, even with overdriving the camera speaker!

[BLT]

Based on the drawing, here's what I came up with for a schematic. This is, of course, unverified so weigh in with any corrections that need to be made.

[M Fowler]

Looks like a Dr Z Stangray with Octal power section.

[BLT]

Does, doesn't it. I started tracing it out and except for one slight variation I recognized the front end pretty fast, since I've planned on building a Stangray for quite awhile now. You know how it is, things get in the way and other builds crop up. So, it appears to be an octal fixed bias Stangray. No cap to ground off of the EF86 pin 1 grid, and no split rail. If there's a chance, it would be nice to see a voltage chart for this amp. Nice sounding amp.

[WRC34]

Whoah BLT, that's incredible. I am humbled, thank you!!

There actually is a .1uf cap to ground from EF86 pin 1. My apologies for the mistake, I never thought this error would end up being an inconvenience to someone making a schematic.

Funny thing is I never have played or looked at a Stangray. I owned a Route 66 for a while and used what info I could find online for component values for the EF86 - the Vox AC30/4 schematic with an EF86 input tube was also a reference. I may have tweaked some values in the final build.

I'll get some voltages added within the next day or so. Although the PT used is less than ideal, and the amp would be 'perfect' with a power transformer that had a HV secondary of around 325-0-325 or so to allow use of a 5AR4 rectifier.

I am hoping to get a chance to make a better sounding/more in depth demo video soon. Will post it here when I do. Thanks for all your kind words!

[M Fowler]

Yeah also looks like this schematic I got some where using 6V6 power section.