'59 Bassman clone build - success!

[Midtex]

I was unable to find my original post so here's the summary. I wanted to build a '59 Bassman clone using iron that I salvaged from a Heathkit W4M PP 5881 hifi amp. After completing the power supply it became clear to me that the B+ would be too high and the current rating on the hv secondary was too low at 130ma, so I ended up repotting a replacement Bassman PT into the Heathkit can. I wanted to eliminate the separate normal and bright inputs of the original design and use a single input and utilize the extra 12ax7 section as an additional gain stage ala Pignose G40V. I finally finished the amp last week and was able to take it to a friends house today for a thorough test. I am not a player.....  It might seem strange to undertake a project like this when I don't play, but I absolutely love electric guitar and I have a fascination with tube electronics and just had to prove to myself that I could do it! I love single-coil Fenders and bought a G&L ASAT (Tele) a few years ago and one day hope to find the time to learn to play. Anyway, the amp sounds fabulous! Upon initial startup I noticed a fair amount of hum with the master volume turned up. I had put some Sylvania 12ax7's in it since they were the most plentiful and least valuable of my stash. I was thinking I might have a heater wiring proximity issue but first decided to swap the first 2 tubes around and viola, the hum went away! I have since pulled the Sylvanias and put in some RCA's and the hum and hiss levels are quite acceptable. I couldn't be happier. Here are a few pictures of the finished amp. I just need to add a fuse on the hv, label the controls and put on a bottom cover and feet. Thanks for the helpful discussions that led to a successful build!

[woolly]

Nice job, looks great.  Your G&L is a good guitar.

just learn to tune it and learn the E major chord and wail away.. 

[kagliostro]

NICE ! 

you miss only a faceplate

Kagliostro

[DummyLoad]

very nice!

love the color!   :glasses9:

[LooseChange]

Nice job! I like the unique layout and color. Where did the iron come from?

[tubenit]

Well done!

Tubenit

[Midtex]

Thanks for the nice comments. 

Loosechange, the iron is from a Heathkit W4M "Williamson type" hifi amp. I saw push-pull 5881 and figured it had to be a good candidate for a guitar amp. Turned out I had to replace the power transformer with a replacement Bassman type to get my voltage where I wanted it. I bought one that had a universal primary with 125v option and all voltages are great including heaters. The choke is rated 7H and 160ma which is more than sufficient since the plate supply is prior to the choke. The output transformer is not a real close match. The Heathkit design is ultralinear. I just capped the UL taps and left them disconnected. I never could find specs on it, but I'm guessing the primary impedance is probably 8k-10k compared to 4k of the original. I believe the power output is reduced by this change and probably there are changes to the sound as well. My idle plate voltage is 432vdc using the RCA 5R4GY. I have the bias current at 36ma (35ma on one tube and 37ma on the other). I figure that is just shy of 70% for the original 23W Tung-Sol 5881's.

We tested it with a sealed 2 x 12 cabinet filled with Celestion Vintage 30's - parallel 16-ohm for 8 ohms. I will go quite loud, but the separate gain and master volume pots allow you to get into distortion and still keep the volume where you want it. It will still go plenty loud while clean too.  I'm going to take it to another friends house as soon as I can for his opinion. He's a Marshall amp/Les Paul guy so it will be interesting to get his evaluation too.

[softwarejanitor]

Are you going to leave the top wide open to the breeze like that?  I know those wacky tube Hi-Fi guys think that is stylish...  but it seems awfully dangerous for a guitar amp!  First time you swing your guitar's headstock around (it happens) when you are playing and want to adjust the tone or something you are likely to break a tube...

[Midtex]

I hear you. The chassis is a Hammond 16" x 8" steel and they make a cage top for it. I will probably buy one but I sure like the looks better without it.

[drumslinger2000]

nice little amp there Midtex.  well done sir.

[softwarejanitor]

I hear you. The chassis is a Hammond 16" x 8" steel and they make a cage top for it. I will probably buy one but I sure like the looks better without it.

Maybe make your own cover out of Lexan (better for heat than acrylic/plexiglass)?  Make sure to drill plenty of vent holes though.  Then you could see the tubes glow and still have some protection against breakage...

[Jack_Hester]

I finally finished the amp last week and was able to take it to a friends house today for a thorough test. I am not a player.....  It might seem strange to undertake a project like this when I don't play, but I absolutely love electric guitar and I have a fascination with tube electronics and just had to prove to myself that I could do it!

I only recently picked up my old Harmony H76, again, after not having it in my hands since the early to mid 80's.  I also have a '59 Bassman in the works.  I really like the simplicity of your build.  Mine won't be so simple, but I'm enjoying every minute that I spend on it.  I built a Champ in an EVJ chassis a couple weekends ago, and took it to Church to leave with our music leader and his son, to give a workout.  It has a very clear sound, almost a ring to it.  I'm looking forward to what they have to say about it.  Hope my Bassman turns out as good as yours.  Keep doing what you're doing.

Jack

[Midtex]

Thanks Jack. I am relying on band members at church as well to help me evaluate my project. I run sound up there so I have lots of musician contacts. I have some transformers that I pulled out of a "Voice of Music" high-end stereo console that was push-pull EL84's. I think that will be my next project. The OTs are 4ohm secondary only. Might make a nice combo amp with 2 x 10". Anybody got any suggestions for a circuit to use?

[punkykatt]

Wow! nice amp. It dosent get anymore point to point than that.  Did the size of the chassis influence your layout? Great job.
Punky

[Midtex]

I wish I could say that I had a master plan from the beginning, but I really didn't. I basically took my time and experimented with different layouts until I found what seemed the most logical. I had the 3 transformers and lined them up on the table at a reasonable spacing and then looking at the Hammond chassis catalog I decided that 16" x 8" would work best. I wanted things spread out enough to allow good heat dissipation. From there I started experimenting with the transformer arrangement while looking at the schematic until I found what I thought would keep the wire runs the most logical. I did the same thing with the pot placements. BTW, the controls are left to right - Power, Standby, Input, Gain, Master Volume, Treble, Mid, Bass, Presence, Power Indicator (6.3v). I found a couple of 13 lug terminal strips at the local electronics distributor and since I didn't want to drill any unnecessary holes in the chassis, I found places where those would fit under existing bolts. that's why you see the odd slant cut to the capacitor board. It is running parallel to the terminal strip that I used to build the different power supply voltage drop/filtering stages.

I initially ran the 6.3 heater wires directly to the first gain stage and then after reading more online I found a recommendation to make the most sensitive stages last in the wiring so that the current is lowest in the run at that point. That made a lot of sense to me. The star grounding page on the Aiken Amps website provided extremely valuable information as well. It's hard to tell from the picture, but the only point that is grounded to the chassis other than the AC input jack is the rectifier tube socket. Everything else runs to that point. I used a piece of 14 gauge copper wire and formed a buss bar around the 3 12AX7s and used that as the comon ground point for those circuits and then ran a single piece of wire from that buss to connected it to the "star". It must have worked because it's pretty quiet.

I also used shielded coax cable (with only one end grounded) for the longer cable runs in the first stages to help eliminate picking up stray signals.