Hammond AO-35/A0-29 Bass amp project

[ZenithNut]

Hello,

I have a Hammond Ao 29 and Ao 35 tube amp chassis I pulled from an organ I've gutted. Theyre complete with made in holland RCA tubes. I would like to use one or the other to make a Bass amp. I dont play guitar but I would love to make my own bass amp. Besides the amp and tubes, I grabbed the speakers as well. I still have the unit in the van so I can pull the on/off switch to reuse that for my project. I also have a dead SWR working man amp I can salvage an input jack, fuse holder, and possibly the potentiometers.
I want to ask a little help with what spec volume/tone/bass controls I need to source. Does the whole tone stack (vol/bass/tone) go inbetween the first and second pre amp tubes?
I want to use the AO 35, as it seems more straightforward. Its a 6bq5 PP amp. Surely this can handle being a bass amp? Or am I better off using the AO 29 with its 6v6 PP configuration?
I imagine the guitar amp conversion right ups will be my mirror save for some bass specific discrepancies, and what those differences will be ie (what I need to change to be a bass amp vs a guitar amp conversion)
Output transformer. I already have seen mention in the guitar conversions the stock Hammond might not fit the bill, and others say its fine. Any suggestions?
Any chance on reusing the rather generous Hammond speakers? Or should I put a nother 1/4 jack to go into a cab?
Thats it for now, the Hammond was free and very complete. Scored a lot of good tubes and components so I want to make something with it. Cheers

 

[ZenithNut]

Doing a little searching I think I'll want to essentially clone an ampeg PF 20T. Its only around 20 watts, and uses 12ax7/6v6 so the AO 29 it is! Should I get the same OT as the ampeg unit?

 

[trobbins]

Perhaps start with the basics and point to a schematic of what you want to modify (AO 29), and to any relevant restoration thread of that chassis.

 

[Mr Boochie]

I don't know if you plan to gig with this amp, but a push-pull 6V6GT may not have enough beans. If you want to have a tube bass amp, maybe a clone of the flip-top Ampeg B-18N is a better choice?

A good bass amp needs to be clean. It needs hefty transformers. A front-end and output sections with good headroom.

I'd prolly go with solid-state power supply rectification. And you can use 7581A tubes in place of the harder-to-find 7027A.

The Hammond 6V6GT amp would make a nice guitar amp.

 

[gadget73]

are the transformers actually any different? Hammond sometimes used the same one with multiple tubes.

a 6bq5 and a 6v6 are very comparable power wise, usually can get a bit more from a 6bq5 but both are ballpark 15 watt amplifiers if you aren't using "if lightning strikes the pole while being measured" sort of numbers.

For limited frequency response its probably a perfectly fine transformer, they don't tend to be great hifi units. Limited on the top end mostly but some of their smaller tone cabinets also had small transformers that don't go way down either. Basically on par with not so awesome console stereo iron. I have a set on a console pull, it does OK but won't do way up or down despite all the torturing I've done to extend the bandwidth. The chassis is just too small to fit anything awesome on it though.

 

[dcgillespie]

With a capable OPT, 20 watts will make for a good practice bass amp, but even small gigs will generally need a little more power. Either the Ampeg as Boochie mentioned, or a classic Fender Bassman will make for a great tube amp arrangement for either practice or small gigs.

Dave

 

[Mr Boochie]

With a capable OPT, 20 watts will make for a good practice bass amp, but even small gigs will generally need a little more power. Either the Ampeg as Boochie mentioned, or a classic Fender Bassman will make for a great tube amp arrangement for either practice or small gigs.

Dave

Or he has to mic the amp to the house system. I like 50 watts to start, with a bass amp. Plus an efficient 4x12 cab, or 2x15.

 

[arts]

If you want to have a tube bass amp, maybe a clone of the flip-top Ampeg B-18N is a better choice?

I'll second the Ampeg B18N. I like 'em so much I've got a pair.

My preferred living room rumbler;

 

[ZenithNut]

Alright, what great feedback Ive gotten!

This is just for my own practice purposes. Like I said, I dont really play guitar but maybe Ill make the AO35 into a guitar amp as an excuse to get a strat to riff on, and the AO29 will be my bass amp build. I dont get and likely will not but If I need more I have an SWR set up. Forget the model but it uses a 12ax7 pre amp.

I don't know if you plan to gig with this amp, but a push-pull 6V6GT may not have enough beans. If you want to have a tube bass amp, maybe a clone of the flip-top Ampeg B-18N is a better choice?

A good bass amp needs to be clean. It needs hefty transformers. A front-end and output sections with good headroom.

I'd prolly go with solid-state power supply rectification. And you can use 7581A tubes in place of the harder-to-find 7027A.

The Hammond 6V6GT amp would make a nice guitar amp.

This is a great suggestion! Itd be easy enough to use a diode bridge in place of a 5u4. I can strip down the chassis to sockets and start over to clone this B18N. Looks like maybe the 7581A will use the same sockets? Ill have to trade out the 12ax7 sockets for 6SL7 sockets but thatll mean more 12ax7s to roll in my fisher or 175 maggie amp. Ok I like this

 

[Mr Boochie]

Alright, what great feedback Ive gotten!

This is just for my own practice purposes. Like I said, I dont really play guitar but maybe Ill make the AO35 into a guitar amp as an excuse to get a strat to riff on, and the AO29 will be my bass amp build. I dont get and likely will not but If I need more I have an SWR set up. Forget the model but it uses a 12ax7 pre amp.




This is a great suggestion! Itd be easy enough to use a diode bridge in place of a 5u4. I can strip down the chassis to sockets and start over to clone this B18N. Looks like maybe the 7581A will use the same sockets? Ill have to trade out the 12ax7 sockets for 6SL7 sockets but thatll mean more 12ax7s to roll in my fisher or 175 maggie amp. Ok I like this

You will need higher current-capable power and output transformers. A 7581A will work in a 6V6GT socket, but will draw more current and need higher plate voltages than a 6V6GT amp was designed for.

Man, if you have an SWR bass amp (I have a Strawberry Blonde acoustic amp), I'd stick with that for bass use. Just get a smaller cab if space is an issue. Like a 1 x 15inch.

SWR made some great bass amps. I've played on the Workingman's Pro 15.

It's just a thought.

 

[gadget73]

load impedance is probably wrong too. 6V6 are typically around 10K load, a 7581 or other 6L6GC class tube usually wants something closer to 6K. They can be used at higher impedance but it loses power, and you probably want to knock down the screen voltage so its not abusing the tube so much.

 

[ZenithNut]

You will need higher current-capable power and output transformers. A 7581A will work in a 6V6GT socket, but will draw more current and need higher plate voltages than a 6V6GT amp was designed for.

Man, if you have an SWR bass amp (I have a Strawberry Blonde acoustic amp), I'd stick with that for bass use. Just get a smaller cab if space is an issue. Like a 1 x 15inch.

SWR made some great bass amps. I've played on the Workingman's Pro 15.

It's just a thought.

I just really want a tube based amp for my own use and dont have the money to buy one. But making one is totally on the table. If we need to get upwards of 500vdc of B+ I wonder if I can just snag a PT from a 60s tube color set. Thatll have the high B+ for the plate voltages and a 6.3vdc secondary for the filaments.

 

[billyz]

I think either will make a decent amplifier. Low watts of course, but good tone. For bass you will want a really good speaker cabinet. Probably ported. I use an AO35 converted for guitar into a 12" Theil type cab . Sounds great and I would not be afraid to plug my bass into it for low volume household chores.

 

[Mr Boochie]

I just really want a tube based amp for my own use and dont have the money to buy one. But making one is totally on the table. If we need to get upwards of 500vdc of B+ I wonder if I can just snag a PT from a 60s tube color set. Thatll have the high B+ for the plate voltages and a 6.3vdc secondary for the filaments.

Restore a vintage Fender PA100?

 

[trobbins]

What power transformer models on the A0-29 and the A0-35 chassis ? Eg. 003- 024956. Perhaps worthwhile identifying what the HT Vac levels are, and what options you have for playing with the B+ level to suit output stage tubes.

 

[billyz]

I recall about 330VDC on the Plates. Perfect for the 6bq5's. Probably why they last so long in these AO35 amplifiers.

 

[Mr Boochie]

I recall about 330VDC on the Plates. Perfect for the 6bq5's. Probably why they last so long in these AO35 amplifiers.

That would make a great basis for a guitar amp. Prolly not the huevos for a bass amp.

 

[billyz]

That would make a great basis for a guitar amp. Prolly not the huevos for a bass amp.

With a good efficient speaker cab , I could use it for a bass rig at home or even an acoustic coffee house gig. Probably get a little dirt at times , like an old B15.

 

[gadget73]

Also depends on the tube, some run lower voltage and high current. Different output transformers with lower impedance to get use of that current capacity tho

6cw5 for example runs 250 volts and a pair can do 25watts, but its a 3K transformer and 180mA at full scream

 

[trobbins]

It seems the A0-29 and A0-35 were separate chassis in the earlier version of the M100 organ only. The service manual shows the wiring to those two chassis in Fig.49 (p.73).

It seems that the A0-29 was the main chassis in the M, M2, M3, M100 organ models, and the M100 used the 003-024157 PT (T7) and the 003-024111 OPT (T3), and used a 5U4 rectifier and 6V6 PP.

It seems that the A0-35 was in the M100 organ only as the Reverb amp, and used the 003-024956 PT and the 003-036552 OPT, with 6BQ5 PP, and either 5U4 or 5Y3 rectifier.

Fig.46 shows the A0-29 power supply schematic, with 320-0-320V into 5U4 and 140mA 320Vdc output plus what seems like -22Vdc of back-bias. Apart from PT effective winding resistance, there was an additional 30 ohm before the 40uF first filter cap. So just based on using an ss rectifier, and an alternative to back-bias, and a larger first filter cap size without any limiting R, and unloading the PT from many valve heaters and some B+ feeds, the available B+ could be circa 360V, or even higher if using the 5Vac to power a step-up dc/dc for an extra say 30-40Vdc.

Fig.37 shows the A0-35 power supply schematic, with 290-0-290V into 5Y3 and ?mA 305Vdc output using a 40uF first filter cap. 6BQ5 idle was 85mA total PP.

So just for grunt, the A0-29 amp with good 6V6 may well get 15W, up to 20W, depending a bit on the PP impedance and how high the B+ is (Fender run up to 400V it seems), and perhaps some other aspects. The A0-29 amp circuitry also likely needs the least amount of modification to make a clean signal path.