bluesman
Be gentle with me, this is my first post here.
I have always liked the sound and tone of the Fender Champ guitar amp but the shortcoming of this amp is the lack of power, being only 4 to5 watts or so. For the longest while I had been toying with the idea of building a single ended Class A amp like the champ only with more punch while not sacrificing the tone and warmth. About a year ago I constructed an early 50’s Champ clone amp for my friend Roy and the beautiful sound grabbed and shook me hard. I don’t play much and most of what I do play is bad, so this seemed another perfect incentive to self motivate, but the idea was still in a vague planning stage and very much in flux. Sometime during the middle of February I had a long overdue long distance talk with my friend Bernd in California. He told me about this little Bogen amp that had beautiful tone, 10 watts of power and much appreciated by blues harp players. This was intriguing; I had the valves (no bases or output transformer) and decided to put my single ended amp plan in motion. First I had a chassis made at little expense and then started the layout and planning stage. It was at this time I realized the 7868 valves I had were well used and ordering sockets would be timely and I am not a particularly patient person. So I decided to opt for using either a 6L6GC or 7591A valve. I chose the 6L6 just because I did.
Initially, I tried a 375 0 375v power transformer but the current was not adequate for the 6L6. So, in my infinite wisdom I used a massive 300 0 300v transformer but could not get the plate voltage I wanted so I had to use this big 450 0 450v mains transformer that satisfied my requirements. A lot of drilling and cutting took place here. The next problem was finding 600vdc electrolytic capacitors to filter the AC. There ain’t none in Bloemfontein so I had to double up with 450v caps, not a train wreck but something I would have preferred not doing, With over 500vdc on the B+ rail, a 5AR4 rectifier was less than adequate so I used a GZ37. These are rare and expensive, but it worked for me.
But, with over 500vdc on the anode, I had to rethink the output tube. A Tungsol 6550 seemed appropriate. My decision was made. I used an ECC83/12AX7 dual triode for the preamp driver and thought things were going fairly well.
At this point a single ended output transformer became critical. My friend Oliver worked out some insane math regarding turns ratios and suggested a 4:1 winding might suffice. I found a 220v primary / 7v secondary transformer that was close and decided to go with that. This required removing all the laminate and facing the E’s and I’s in a common direction with an insulate between them to prevent field saturation. I did this and it was a pain, but it worked, less output than I wanted, but it worked.
While trying to talk myself into believing that I could live with the diminished output, I also had the problem of adding a reverb tank. 20 years ago I stripped a 4 spring Accutronics reverb out of an old Hammond organ someone was tossing, saving it for all this time for just such a project. Per the Accutronics website the model number indicated the impedances to be 150 ohm in and 2250 out. Because of the limitation of the chassis size and the many transformers tried and thought to be final, I needed to use only one tube for the reverb driver and amp, an ECC83. I once owned a Gibson Hawk guitar amp and loved the reverb so I thought to loosely copy the design. WRONG! My reverb tank simply did not work. I could get it to twang if I thumped it, but no signal went through it. On my oscilloscope, the input signal lacked the amplitude I thought necessary. With Bernd’s sound advice, I decided to drive it hard. Still no joy. I tried over 20 transformers to mate this tank. I made a few enquires and found this old scoundrel in town who does valve amp repairs and he agreed to let me try one of his used tanks before paying the ridiculous sum he wanted for it. It barely worked with one old radio interstage transformer and was only a 3 spring job, 1500ohm in and 2350 ohm out impedance. I took Bernd’s advice, drove it hard and I now have a reverb, not the ultimate deep surf I’d prefer but its good. In the meantime, Oliver came up with a solution for the 4 spring tank but I’m saving that for the next project, a Fender Bassman made from what Oliver calls The Mosque amp.
My son Martin came to visit from his studies in Cape Town shortly afterword. With him he brought a Hammond single end output transformer given to me by a friend named Jean. This output transformer is rated at 80ma and my anode current measured 74ma. I have to add that I tested the 6550 output tube, then a NOS KT88 Chelmer tube and finally a GEC TT21. The 6550 drew 64ma, the Chelmer 70ma and the TT21 74ma. The TT21 knocked my socks off, goose bump tone, rich and warm sounding. At this point I could use a smile. I used the new Hammond transformer and noticed an immediate output gain, but with only 520vdc on the anode the amp is limited. Using Oliver’s advice, I changed the cathode bypass resistor on the TT21 from 330 to 270 ohms and now I see exactly 80ma of current on the anode, reaching the limit of the output transformer and suspect I’m getting about 15 watts RMS. It’s good enough. I am going to have Andre build me a cabinet and use a Wharfdale 12” die cast speaker. I’m finished with all the refinements, many of which I didn’t not mention here and after 3 power transformers, 2 output transformers and countless reverb transformers, I’m done.
I should also add that I have over 14k valves/vaccum tubes in my collection so this was never an issue. Not sure how to post images here but if interested, I'll email them on request.
Peace Out, Mark
Karel-Mars
Nice one Mark :goodtimes:
It is great to follow your thinking. You seem to be very comfortable with tube technology. It seems the Hammond 125ESE does the job hey?
A question: did you bypass the cathode resistor on the first triode? What value volume pot did you use? Also: what size/type coupling caps did you use? Size of output tube grid leak resistor?
You do not mention a choke? What size/rating resistor did you use to smooth the screen grid supply? Any hum?
A photo of the power supply etc. would be lekka :rolleyes:
bluesman
Hey Karel,
The 1500 ohm cathode resistor in the first half of the first triode is stand alone. Used a .02 coupling cap for the single pentode and a .02 between both sides of the first triode, grid leak is a 220k. The choke meters ar 200 ohms not sure of henrys? It's just one I had lying around that fit under the chassis, barely. Used a 100k on the screen grid supply.
There are 2 schools of thought on guitar amp power supplies. One is that overbuilding them to prevent bottom outs prevents all the nasty but good harmonics musicians like. Of course the opposite leads to hum, motorboating and very unwanted distortion. Middle ground is what seems to do the job best.
The power supply train is 520vdc 30mf, choke 20mf, 47kohm 20 mf, 5kohm 20 mf with absolutely no discernable hum and quiet as a church mouse.
If you email me I'll be happy to share a couple of pics and a schematic that is terribly drawn and not 100% accurate because I made so many changes and am dyslexic.
Forgot, both volume and tone are 100m linear
Mark
MikeM
100m linear? 1M linear? Might be worth replacing the volume with an audio taper.
Really interesting build.
One thing I can suggest is looking at a more efficient speaker.. You're fighting the typology of a single ended amp, when using a more efficient speaker could make an almost equally significant difference to your output.
bluesman
Hey Mike,
Thanks for pointing out my error, Its only noon here and I promise I havn't been drinkin beer yet. The pots are 1 meg, not 100, don't know how I did that typo! The wharfdale is an old speaker, very old and I'm sure it came from some 50's bass reflex cabinent. Don't have an SPL dB rating but suspect it is reasonably efficient. I would experiment with a more efficient speaker, but this is Bloemfontein, Fort Apache and I'm an American who's only been living here 6 years and still kinda green, in the figurative sense. I wish I could source one easily.
How do you post pics here? Is an image hosting site necessary or is it possible to upload?
Mark
I-M-Biased
Hi Bluesman
You woke me out of self-induced hibernation with that magical name "TT21", I have 2 old RCA broadcasting monitor amps with twin TT21's in the output stages running in class A mode. Hope you wear thick rubber gloves when working on that amp of yours, those top caps have a habit of biting. ?
