KEG-EAR 6L6-KT88 PP AUDIO AMP
My focus of tube amps has been on the use of the Compactron tubes for both input and drivers. My skills are in PCB design and toner transfer fabrication, not talented in clean point to point construction that so many of you exhibit. I received a message from Kegger (AK site) to see if I was interested in a Dynaco 70 style circuit and components that he had been working on. I was intrigued by the circuit and modified one of the Compactron board designs to this new circuit keeping the same mounting hole and tube locations. Thus the name on this amp after both of our inputs is the KEG-EAR AUDIO AMP.
My goal was to design a PCB that would work for both ends, the cheapest version possible and on the other end, KT88's and big iron. (like Millet's big red board with sweep tubes) I wanted to create a very simple construction with astounding performance that a newbee could do and be proud of. There are so few simple designs available to get started with. This is a summary of over a years work and evaluation condensed to keep from rambling on.
The power supply caps are stacked for extended B+ voltage. For lower voltage stacking is not necessary and the bridge resistors could be left out and one cap position jumpered, smaller caps used.The iron on hand allowed for 300 to 480v B+ and output iron was 8k-8 25watt to Hammond 1650p 6k outputs at 60 watts. I had one Hammond 4.3K on hand.
It is designed, ironed, etched, plated, drilled and fab'd. I've tried SV6550-C-, EL34 and Russian 6P3S output tubes, for the bottom end it would be the $20 10lb'r with the $4 output tubes 6P3S. Started out playing in UL, Triode mode later. The design goal is simple, keep it simple with minimal phase shift and feedback only if necessary. I left provisions for a dominate pole between the first and second stage if needed and like the ST70 feedback cap again only if needed. I will admit to being a little sick, I put an input coupling cap on the PCB. I still play vinyl and have a little turntable rumble to filter out on some records, and it is cheap insurance if there is an unwanted DC bias on the input.
Power supply is on board as well as a 50,100, 150 volt zener reference, I use the 150 to bias nixie tubes and the 50 volt can be used to raise the heater supply.
The cheapest version can use the 6P3S tubes with B+ from 300 to 365V. Suggest 40 watt iron to maximize power while helping to control cost, I like 6K for more class A operation. If you are going for lowest cost the 8K-8 25 watt transformers could be used in triode mode only(no UL taps).
Wife was out of the house, so I cranked it up with the 6P3S Russian output tubes. Changed the supplies a little with an extra chunk of iron to make B+ to 365V, 2nd stage to 250v and input at 205V. From comments made on different forums, the 6P3S start to loose life over 375V , I had them biased at 45ma, in UL mode.. Very pleasant until it runs out of steam. With my speakers and room size more than ample power, speakers, woofers start to complain first, heart beats dark side of the moon...
Changed out the tubes to Svet SV6550-C, added a couple of 6.3 volt windings to boost the neg bias a little to throttle them back to 50ma. Also run at B+365, Second stage 250, First stage 205 volts. A little warmer sound, fuller with tighter bottom. Again volume can drive you out of the room prior to distortion setting in, speakers, woofers start to complain first, ya, the heart beats...
Installed .33 Russian PIO coupling caps on the left side and boosted the B+ to 450V in triode mode.. 5881's were first for many hrs. I still have the old Hammond 6.6k iron connected. 450 volts sounds better than 365 which was better than 300.... Switched to the SV-6550-C- outputs, after many hrs, still my favorites for this amp. With resistive only GNFB, I can not tell the difference between the 1u and the .33u PIO's. EL34's were OK, from China. I did not have a set of EH's but I'm thinking of getting a set of EH 6CA7's.
The B+ was boosted by adding a toroidal 120vac @400ma output winding to the 10lb'r output. The diode bridge is good for over 800V and the caps are stacked, added B+ delay by a 6CG3 Damper Diode.
For the bias voltage I put a 110 to 12.6CT transformer on with half, the 6.3 to CT hooked up to a 6.3 volt winding and the 110v winding to the bias diode bridge through a 1.2K resister. Instant -90volt DC for the bias circuit..
So far the PCB has been very stable and flexible to component selection.
My second 4.3 k Hammond arrived today, so I did a quick swap to see if the 6P3S-E/5881's could handle them, nope!
Ok at low levels but they ran out of steam, started to smear at higher volumes, needed more feedback.
Svet 6550=C='s are still my favorites. KT88's are very good also with the 4.3K's
My recommendations for output tubes to transformer combo's:
8K IRON = 6P3S, 6P3S-E/5881's, overkill= EL34, Svet 6550=C='s, KT88's
6.6K IRON = 6P3S-E/5881's, EL34, Svet 6550=C='s, overkill= KT88's
4.3K IRON = EL34, Svet 6550=C='s, KT88's
It has been a great project, thank you Kegger!
Pics with and without the bias check on.
That is pretty sweet looking!
You did some really nice work there.
(I'll be asking you for circuit tweaks) :)
Totally interested in a kit... if there were some instructions :)
I created artwork for toner transfer PCB fabrication. The design fits on a 8.5" x 11" sheet so the design can be copied onto hi clay slick paper and ironed onto the PCB to act as etch resist. I use zip-lock bags as etching tanks to contain the mess. The copper is tin plated and then the top layer is created the same way, but the clay layer is removed by slowly rubbing with lava soap, leaving the black toner for component outlines. It is protected by clear enamel can spray. The holes are drilled, routed out by a dremmel tool and the through holes are enlarged and brass eyelets used for wire connect points and mounting the caps on the wrong bottom side.
Are you willing and have the basic tools to so a PCB from scratch? If so I might be able to create a PDF file, copy that could be used as a master to print out and use for PCB fabrication..
Results of toner transfer PCB fab.....
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