HoSo 56 min from Hammond AO-39?

[bmccowan]

Hello,
After finishing the 6G2 circuit, instead of building the cab, I went back to finish a GA-40 Lite I had started in an AO-39 chassis, using 6CM6 power tubes (9 pin tubes similar to 6V6s.) Playing it reminded me how much I like 5879 preamp tubes. I bought a 6-pack of AO-39s a ways back and have a couple left, after also building an Ac-15 Lite and a Supro 24 Lite. I am thinking of building a HoSo 56, very minimalist in one of those. No MV, and no FX loop, just a straight ahead amp with the 5879>12AY7>2 6BM8s and TMB. I know that even with a 5Y3 I will need to scrub off some voltage, but other than that it seems that the Hammond iron should work.
But before I sketch out a schematic and layout, I thought I'd seek some opinions. Especially from Tubenit, who is a fellow fan of the 5879 and Sluckey who has built more than a couple of amps from AO-39s. All others are welcome too. So, is this a good/workable idea? Or should I just build a Hammond B-3 up from that chassis?  Speaking of B-3s, I've seen 3 bands recently with B-3s, and there is no better sound on earth!

[tubenit]

Looking at Sluckey's original A039 schematic,  it shows 315v on the tubes using a 5U4.  So 315v divided by 1.2 = 262v


So, assuming the Hammond PT is around 262-0-262 (or there abouts) IF you used a 5Y3GT then I think you'd be fine and have voltage low enough on the 6BM8 plates.  262 X 1.1 (5Y3GT) = 288v

So, the attached schematic is based on my current HoSo56 minimalist which has an active FX loop using a mosfet for the CF and it has a PPIMV.  So, I removed the active FX loop and the PPIMV in this schematic.  Of all the HoSo56 amps I've built, my current one is my favorite.   (there is an editable SCH version)

The only thing you might want to change is the dropping resistor between node C over to node E to keep the voltage on the 5879 reasonably low  (140v-160v range).  You might try a 4.7k to 8.2k ………… OR just go node C to node D to node E instead.

I strongly recommend having the passive FX loop in your amp. Two jacks don't cost much and delay and reverb with the 5879 tube and 6BM8's sound lovely, IMO. I would anticipate that you would find a passive effects loop works quite well with this.

With respect, Tubenit

[sluckey]

I've only worked with one AO-39 so you have more experience with those than I do. 
 
The original AO-30 schematic shows the PT is 300-0-300 @ 140mA and line voltage at 117VAC. The circuit used a 5U4 with a 50Ω and 200Ω between the rectifier and the plate node to provide 325VDC to the OT center tap. That arrangement gave 315VDC on the plates of the cat biased 6BQ5 PP power amp.

In 2011 I converted the original to a Vox AC-15 lite using the original chassis. With line voltage set to 120VAC that same PT puts out 315-0-315. I used a 5Y3 and a 250Ω resistor to provide 309VDC to the OT center tap. Plate voltage was 305VDC.

In 2015 I scrapped that amp and used the PT and OT to build my Dual Lite amp in a new chassis. With line voltage at 120VAC the PT still put out 315-0-315. I used the same 5Y3 but this time I did not use the 250Ω resistor. This provided 325VDC to the OT center tap. Plate voltage was 318VDC.

So, your 6BM8 amp will likely be a lighter load than the 6BQ5s (EL84s) and this will cause your B+ to be higher than the voltages I just mentioned. But I think you can get the B+ down to a safe value by using a 5Y3 and a series dropping resistor. May have to use a 500Ω (or larger) rather than the 250Ω I used in my original conversion. Here's my conversion schematic to use as a guide for your power supply...

     http://sluckeyamps.com/hammond/hammond_ao39.pdf

This should be a fun project. Looking forward to see this project. That AO-39 has some nice iron for small simple amps.

[bmccowan]

Tubenit & Sluckey - Thanks so much. As usual that was more help than I have a right to expect. And I'll take your advise on the passive FX as I have the jacks and there is space on the back panel. I'm away from home right now building a cottage, but when I get back I will put a 5Y3 in one of my AO39 derived amps and see what I have for voltage, as I jut picked up a couple of nice NOS RCA 5Y3s. My recollection is that I have a 5Y3 in the AO39>Supro build and I have around 300 on the plates. I followed Sluckey's AC-15 Lite schematic for the power rail. As to Sluckey's comment on my experience with AO39s; ok, but a lot less knowledge over here!
My biggest struggle with these little Hammond Organ chassis is doing point-to-point. The layout does not lend itself to turret boards, but I think I will try adding some additional terminal strips to keep things organized.   

[tubenit]

Sluckey gave you some good advice about B+ and voltage! IIRC, the first  6BM8 amp I built had 305-310v range on the plates. It worked great with no issues.

with respect, Tubenit

[bmccowan]

Thanks - that's good to know that I don't necessarily need to get below 300. As usual the max V ratings are somewhat conservative I'm guessing.

[bmccowan]

Hi,
Well I scrounged some parts, found some time and built the amp; well enough to play it anyways. Tubenit, you are right, it sounds great! For other AO-39 conversions I have gone p>p but with so much going on around the ECL-82s, I found a narrow turret board and mounted some terminal strips to keep what's left of my sanity. I finished wiring it a week ago but had numerous issues. Wanting to find and correct them myself, cause I learn better that way, I soldiered (soldered) on, first using the circuit trace recommendations here and then using a signal generator. I had a misplaced connection, a faulty coupling cap, and lead dress issues. The lead dress issues were the hardest, and maybe its squeezing all into a small chassis and/or the high voltage and/or a higher gain amp than I am used to, but I had a hard time and ended up flying one of the leads to the OT up out of the traffic.
I still need to wire up the FX and I left out the boost switch. And I need to clean up some of the wiring after so much lead dress testing.
It's brighter than I like - suggestions?
Voltages came out right where Tubenit and Sluckey predicted - thanks!!
Attached is the as built schematic (a few minor changes) and a low quality pic.

[shooter]

It's brighter than I like - suggestions

try a 10uF on V2's cathode instead of the 2.2
try .1uf in place of the .047 on the bass pot

both could be done with gator clips, use another .047 in parallel with the existing one, just hang a 10 in || with the 2.2uf.
might increase hum/noise but you should still hear if you're moving in the right direction, then you can solder

[bmccowan]

Thanks Shooter. I'll give those a try.

[tubenit]

I'm delighted the HoSo56 Minimalist sounds good to you!  Surprised to hear that it sounds bright to you though?

I am wondering what speaker you're using.  Mine is very well balanced and has remarkable bass response from such small power tubes, IMO.

The 5879 tube is somewhat chimey.  Have you built an amp with a 5879 in V1 before?  Maybe that's some of what you're hearing?   Try Shooter's suggestion with the increased cathode cap.

With respect, Tubenit

[bmccowan]

Hi Tubenit,
It sounds great. And it does have good bass response. I have Tinnitus and am sensitive to higher pitch. So its not a criticism, its me.
5879 - yes I have an original GA-40, and have built: a non-trem GA-40, a 5879 version of a Supro 24 with 6973 power tubes (you commented nicely on that build), a 5879 version of a Z-28, and a 5879 version of Sluckey's AC-15 Lite after deciding I did not like the EF86 in either of those amps. I'll rebuild that AC-15, as I really don't care for the EL-84s.
So ignoring the fact that I just revealed that I'm insane, I love that tube - and the 6SJ7 too!
It indeed could be chime that I'm talking about - I have not tried different 5879s yet and should.
With P90s I set the treble on about 3 and rolled the treble back some on the guitar and got the balance I liked. but as soon as I finish an amp I tend to tinker with it. That seems to be a common affliction.
I will be trying Shooter's recommendations and reporting back.
Oh - the speaker is a vintage Jensen P12Q.

[bmccowan]

I tried both of Shooter's suggestions and like the increased capacitor value on V2 cathode. The tone cap change not so much. I also like lowering the voltage, which I did with a Variac. I may change the 500 ohm first dropping resister to 1K, I think I have a 12 watt on hand.
Also I looked at some of your old posts Tubenit, and my amp is not so quiet. Of course I have a lot crammed into a small space and had to fool with lead dress a lot. Touching the lugs of the volume pot with screwdriver changes the noise but in differing inconsistent ways, and the leads at the vol pot are pretty darn sensitive to dress. I guess I could use shielded cable there, but I'd rather fix a problem than band-aid it.

[shooter]

I could use shielded cable there, but I'd rather fix a problem

I believe Belden makes a boatload of money because that is the fix 

[bmccowan]

Ha! So I can feel better about using some shielded cable to quiet things down. Will do, even though I hate working with the stuff. Hmm, where is the emoji for whining?

[bmccowan]

Noise solved. I got back to this project after some travels. Shielded cable helped some, but not enough. I also improved the preamp ground buss - also helped a bit. I then noted that the standard Hammond routing of the PT high voltage secondaries brought them very close to the pins of one of the power tubes. I re-routed them and now the noise floor is good. This is a great sounding amp; everything that Tubenit says about it is right on. Much louder than I anticipated. The grit in low notes is particularly pleasing, especially with P90s. A bit of clean-up and labels and knobs for the pots and on to the next project.
Thanks all

[bmccowan]

I've been playing through this amp some and it sounds great at lower volumes. However, cranking it up, it gets noisy and harsh. Because I have limited space in the Hammond chassis, there is no MV or trim pot. I have tried different tubes (_U7;_T7) for V2, but the 12AY7 sounds best. I also tried increasing the grid stopper to V2 from 33K to 56K; not much help there. So, I am thinking a grid stopper or voltage divider downstream of V2. Or changing the voltage divider values where the MV would have been. The schematic I followed is the final one that Tubenit and Sluckey collaborated on. Suggestions are appreciated!