Building a Dave Gillespie SE-1 Amp

Many of you have been following the once again 'incredible' work of Dave Gillespie on his little Magnavox 8604-00 'Flea' amp. If you haven't, and want an interesting, informative, educational, and entertaining 'read'..... go seek out the 'Magnavox Flea Power' thread of Dave's. But, be warned: If you're like me, you'll probably want to go out and build one of these for yourself.....which is just what I'm 'trying' to do.

So here I am in this thread rather closing the barn door after the horse has left. Why? Well, my amp is 'built'.....but not right yet. I have some trouble-shooting yet to do, as I've got a 'nasty' noise problem; not your typical 'hum' issues I've had in the past. In fact, the amp is DEAD QUIET!! My amp has a rather aggressive 'scratching' kind of noise. Much like an arcing across a tube socket or something similar. At low volumes...VERY low.....it doesn't act up at all. But, turn the vc just a little and you start to hear the noise on the dynamic and bass passages. Turn it up more and it's distorting all over. VERY annoying, not to mention disappointing! This clearly points out the difference in 'repair-refurbish' vs 'scratch build'. Were this once a 'working' amp, I'd be relatively sure that I was looking for a failed component. However, as a 'scratch' build..... I've not got to go back and see if I followed the road map (schematic) correctly. To that end, I 'think' I've gone over it enough to verify my build IS to Dave's diagram. However, I'm also not so presumptuous as to believe I could not still have made an error. So, in posting the following pics, if anyone sees anything suspect, PLEASE let me know.

The amp started as just a generic version of a little SE EL84 amp. I'm not sure what it came out of, but I confirmed it did have 4 ohm output transformers. Also, it's Power Transformer was a bit more 'gutzy' than that of the original Magnavox in Dave's project. My HV is a bit higher than that of the Maggie....so in a post on Dave's thread he suggested I simply add another stage to the PS filter section to bring my B+ into range. In my pics you'll see a black axial 33uf - 350V cap and a 470 ohm -5Watt resistor. This is that section. My B+ now sits at 270V AFTER the second stage vs the 265 Dave has on his Schematic (found on Pg 7 Post #103 of Dave's thread). My B+1 is 250 vs 245. So..... Onward.........

I pulled the 4 ohm OTs off and replaced them with the one's Dave suggested from AES. Got them mounted, along with a Dynaco choke I had on hand. Specs between the suggested Hammond show it at 1.5H at 56 ohms, 200ma and 400 volts vs the Dynaco choke at 1.75H at 62 ohms, 200ma and 400 V. So, as Dave stated to I believe Derek in his thread: OK to use that choke. Where I think I may have a problem is in my multi-cap. I had to scrounge the parts boxes to come up with something suitable. I found a 3-section cap that was 40-40-40 at 400, 350, 300. The Maggie (and Dave) use a 3-section of 40-40-20 at 350, 350, 300. While mine 'should' be useable, I note that it has a slight bulge in it.....and I'm REALLY skeptical of it's quality. Further, the noise I hear is one I've heard in the past in amps with failed PS filter caps. So..... I'm hangin' my hat on that, and buying a replacement tonight. (would have done it over the weekend by AES-CEDist was closed for the 4th).

If you look at my pics, you'll see a couple places where I had to get creative with some resistors in order to come up with the proper value. ie, I didn't have the 820 cathode resistors of the 2 sides of the 12AX7. So, in combining a 470, 330, and two 10 ohm resistors..... I got my 820 ohms. Is that a problem? I hope not! If so, please let me know. Likewise I had to pair up a couple more resistors to get a proper value....but again, it got me to the correct level vs taking a 'close enough' approach. My only other deviance is the use of a 390pf cap on the bypass of the feedback resistor vs Dave's spec of 300. Could this be my problem? Maybe! I'll get a 300 and 'fix' that issue. But, my inclination is that my problem lies in either A) that PS issue. or B) something I simply did wrong!

So, here are my pics! I will update this after I get the multi-cap and remaining other passive parts and 'correct' the 'make do' stuff. With luck I'll have the multi-cap from CE by the weekend. The passive parts will either come from CE or a local supply place on the North side of Baltimore about a hour away. To conclude, I'm COMPLETELY open to ANY comments; good, bad, or indifferent. And, if anyone sees any kind of problems with my build, PLEASE let me know!! I can't tell you how frustrated with myself I was when I flipped the switch, heard this dead quiet amp...and then got that noise problem. It was like going from 'success' to 'failure' in a span of 2 minutes!! Onward.........

Tom D.

Tom -- This may not be component failure oriented, but one of stability. Consider:

1. Lead dress is important. With a response to 40 kHz, having long coiled up OPT plate leads in close proximity with long input leads is potential trouble for sure. Keeping inputs and outputs separated -- both of the compete amp, and for individual stages -- will help to minimize these concerns. In the Magnavox chassis, input and output leads were well separated.

2. Your layout may also require a stopper resistor at the control grid of the output tubes. Disconnect all leads to pin #2, and insert a 1K resistor between those leads and pin 2. Keep the leads of the resistor short.

While my build was very stable with any input impedance, using a source with a low output impedance will always help to quell any instability taking place. Your description of the malfunction certainly sounds like instability. Based on the description of the can cap you used (bulged out), it no doubt needs to be replaced. But I would certainly address these two areas offered as well.

Let us know!

Dave

Dave,

Thanks for the advice! I'll make those changes immediately. I've not yet ordered any of the other parts....... many things going on here this evening including yet another Thunder-banger! Thought the juice was going to fail again!!

I did receive a PM advising me on the multi-cap situation. Not sure what to do do proceed with that aspect of things.....may end up just using some axial types underneath. I'll keep things updated as I progress (or regress).

TSD

Dave, (or anyone else), the schematic shows 118V at the plates of the 12AX7. I'm measuring 150. I've been all over the 'build' aspect of it..... and checked my 245V supply to the 470K to the plate. That's all spot on. The 150K between the 470K and the cathode of the 12AX7....and the 820 to ground...... all good. I can't see 'where' or 'why' I'm not loading down to the correct 118V? Any ideas? WC

It would also help to list all the key voltage readings.

Whooops -- My proof reading department has been severely disciplined for intermixing pre and post modification data on the schematic. Sorry about that -- you just can't get good help anymore.....

Thanks for keeping me honest Tom -- the plate voltage of the AF amplifier tube should be labeled as 132 volts. Somehow, the voltage I recorded from the original design crept into the schematic of the modified amplifier. I've double checked it (again) for any other errors and all else appears good.

The fact that you're noting 150 volts is really of little concern, almost certainly due to variations in individual tube characteristics. Of the 6 different random tubes I tried, I noted voltages as low as 128 volts, and as high as 142 volts, but a tight average of 132 volts was most common.

You might remember from my thread that this tube is operating with an effectively reduced cathode resistance value, because of the virtual fixed bias platform the tube stands on as taken from the original design. This in turn allows for the characteristics of an individual tube to be more "on display" than it normally would had a traditionally larger cathode bias resistor been used that was returned to ground instead.

This is nothing more than you've also observed between output stages that are cathode biased or employ fixed bias: Cathode biased designs largely compensates for individual tube characteristics, such that most cathode biased output stages have no, and really need no facilities to adjust the quiescent current. The cathode bias arrangement does that automatically. However, the same tubes in a fixed bias arrangement need controls to account for the individual characteristics of the tubes, since no compensating circuit exists to account for that.

It is much the same thing in the scenario with the AF amplifier stage in this design as well. The variations in plate voltage this produces for this stage can be discounted as having virtually no impact on the performance of the unit.

Thanks for helping to catch the error!

Dave

Well, Dave nailed it!!

I moved the OT wires back away from the two input resistors, and added the grid stopper resistors.......and the amp is now functioning what I'd call 'beautifully'!!!

I'm actually astounded that just those two changes corrected this....but I'm delighted! No more 'crackling' or 'buzzing'..... and the amp seems so smooth that I'm just in awe of it! It's playing now on a set of test speakers.....but I think before I end this post I'm going to go swap it to the Cornwalls....... right now!!

WOW!!! If I could use the 'F' word here, I'd use it in conjunction with word 'amazing'!!

While I've yet to go back and hook up one of my original Maggies...... I can honestly say that this amp rivals some of my smaller PP amps in performance. Granted, I'm now on my Cornwalls.....but the bass is deep, strong, NOT flabby or 'farty'...... it's articulate, concise, and in complete 'sync' with the rest of the music signal. It's not just 'there' in the background....but it's clearly supporting the bottom end with all the magnitude of the upper registers. And, speaking of those, the 'highs' in the amp are all the things you'd expect from a 'hi-fi' amp! It appears to be missing NOTHING!! Honestly, I'm really aghast at the difference. Even without hooking up my old Maggie, I KNOW there will simply be no comparison. I can't wait to show this off to some friends!! And...... I can't wait to build my next one!

Dave..... This was an amazing project! BTW, I'm listening to Spirogyra out there, which is no small reproduction task. The bass line is heavy, the vibes and woodwinds are demanding, and the percussion will take it's toll on even the best of amps. This little thing is handling it like any ST-35, or (and I hate to say this) even my 7868 PP amps! I'm dumbfounded!!

Gotta go. I need to sit in front of this for a while!!

Tom

That's great Tom -- as I basically said in my original post, the difference between the original amplifier and the modified one is really amazing -- just hard to believe that such good sound -- and the quantity of it -- can come from such a simple unit.

The lesson with lead placement is a good one. The original design amounted to little more than a "record player" performance wise, and as such, the circuit was so low in performance that lead placement was really quite UNcritical -- just the thing you need for an economical, mass produced lo-fi unit. As performance increases however, then good build practices really come into play quickly as you found out.

Glad it's working well for you -- enjoy!

Dave

Wharfcreek said:

WOW!!! If I could use the 'F' word here, I'd use it in conjunction with word 'amazing'!!

Click to expand...

Based the linked article, I think it's possibly ok to use the F word in the adjectival formula but not so much in the verb form.

http://www.washingtonpost.com/wp-dyn/articles/A22191-2004Jul1.html

Glad the amp is performing.
John

Nerdorama, I had to laugh when I read that link! Being outside of DC you'd think I'd have been aware of that little fiasco....but not so much. However, seems the Supreme Court DID reverse the 'OK' for Adjectival use........ so, not sure if that was 'progress' reversed, or 'justice done'. I'd probably vote for the latter....but that's just me.

Anyway..... amp is working..... I spent the better part of 3 hours listening to it, and even demo'd it for my significant other. She liked it, though she and I have FAR different tastes in music. But, at least she 'gets' my addiction....and leaves me alone to build, listen, and 'collect' as I see fit. Some times I wish she'd reel me in some on the 'collecting' thing.....but....could be worse.

Dave, I think I'm still detecting just a bit of instability in this thing.....perhaps as a result of the 390pf cap where I should be using the 300. As soon as parts arrive I'll swap that out. Beyond that....this thing is a 'Little Demon'! Really an amazing thing! I can't help but wonder what you could do with an SE project around a 7591 or EL34 tube (with the former being my preference ......lol). I plan to wrap this amp up tonight, and perhaps do a few final 'clean-up' details like adding a face plate to the front to cover up the holes and then installing a pilot light. Other than that, adding the 300 pf, and maybe a 'new' set of tubes......... I'm going to call this amp 'done'! And, move on to the next one..... I've got #2 DG-SE1 already in the works!!

Thanks again to all!

Tom D.

The design is hardly sensitive -- as in hair trigger wise -- to stability issues. Just following good layout and best build practices should result in a quiet and highly stable replica. However, the larger value FB cap would in fact reduce stability, since a move in that direction acts to increase NFB at high frequencies, which is the end of the spectrum where any problems -- if they're going to happen -- will happen.

If you have a scope and sq wave generator, you can always check for any potential instability by comparing the waveforms developed against the waveforms I provided under various loading conditions in the original thread on the amplifier.

Dave

Dave, I fear I'm not quite there yet as far as doing a 'scope & SigGen' eval..... wish I was. But, my level of know-how hasn't yet gotten that far yet. In due time though!! Clearly, I'm going to need to advance to using those kinds of diagnostic tools if I ever want to really understand the results of otherwise just 'shoot-from-the-hip' changes.

I do have one question that may seem insignificant, but then with this being a 'hair-trigger' design, maybe it's more important than not. This relates to the use of a 6V pilot light in the filament circuit. As it stands now, I have those 100 ohm resistors from each leg tied together with the junction going to one cathode of one OT...... I believe correctly as per your design. Would the use of a 6V pilot light cause a problem in this circuit....either in additional current draw, or in any feedback created within the circuit?

Thanks Dave.

TSD

Check the first sentence of the last post: The intention is to say that the design is NOT sensitive with regards to stability issues -- if it was, it wouldn't be very stable then now would it!

As long as the pilot light socket is not grounded in any way, then powering that lamp from the positively biased heater winding is a complete non issue.

Dave

OK, got it on the 'sensitivity'. I actually read that as in 'it had hair-trigger' sensitivity. So, I completely misunderstood it. But, got it now. I'll make sure no contact to 'chassis ground' within any pilot light circuit I might install. I have used those pilot lights like you have in yours...... I think they're 117V from RS? But, I'm kind of partial to the old bulb style types with the screw-in jewels. Something about those just says 'vintage' to me.

Anyway....... searching for a PT for 'take 2' on this amp build. All else is 'in the mail' according to the shipper. Damn...... I just realized I forgot about finding a suitable chassis! Oh well......... got a week or so to figure that one out.

TSD

I'm still waiting for all my parts. Once I have 'em, I'll post my build thread.

-D

Derek, what r u using for a PT? WC

WC - What is the cathode voltage of the 6BQ5? Dave showed 5.7V on his schematic.

Wharfcreek said:

Derek, what r u using for a PT? WC

Click to expand...

Edcor XPWR221.

-D