Beginner project: Heathkit A7 into guitar amp

[G-hoppa]

Hi there-- I've been reading the threads here for a while, and it's time to dig in on a project of my own. Hopefully I can get some ideas from the collective experience here.

I just got a Heathkit A-7E, and the build (circa 195?) was so horrific I had to gut it. I tried to mod an old Bell 2122-C hi-fi a bit ago, got in over my head, and made a mess. I figure I'll go back and fix it when I know more about circuit design and have polished my soldering skills. Anyway, I thought that was ugly, till I turned this A-7 over and looked at the wiring job! I'll bet it never worked, and possibly killed the guy who first plugged it in!

So I'm down to a couple trannies and some solid tube sockets. It's got 12V heater taps, and all octal sockets, and I'm thinking of building something based on an early Fender Deluxe design (push-pull, 5y3 rectifier). I know they used 6.3V heater tubes (6SC7s, 6SL7s, 6V6s). I'm not really trying to copy a Fender circuit, just use a similar tube complement/layout/circuit plan.

So here are my questions for you guys:
how hard is it to get the 12V equivalents to those tubes?
Are they around? NOS only?
Are they actually equivalents to the 6.3V tubes (in terms of plate voltages, current and etc.)?

And-- how far would you need to deviate from this schematic to create an amp that does right by electric guitar? I assume I'll need a different tone stack (maybe a fixed mid). In terms of tone, I am open to different sounds, but want to be able to push into sweet distortion.

What would you do to this, given the trannies I have, the 12V needs, and the octal sockets I'd like to keep?


Here's a link to the A-7E schematic on the Audiophool site: http://www.audiophool.cjb.net/Schem_A/Heath_A7.gif

Thanks in advance for any suggestions!

--colin

[Bruce / Mission Amps]

You can wire any two tubes with 6.3vac filaments in series and run them on 12vac.
The filaments would be like two nearly same resistors in series, so the 12v voltage will be roughly divided evenly at around 6v for each tube.
But truthfully those 12v tubes are easy to find in NOS and I would just use them and build your own version of a tweed 5C5 type amp.

[G-hoppa]

Thanks, Bruce. I guess I'll keep it easy and use the 12V tubes. The output tubes were originally 12A6s. Would it be worth using those for a guitar amp? What would be a useful replacement? The OPT has a pair of taps to feed current to the plates on those, and I don't know the voltage (250V would be the max on those, right?)-- so I am guessing if I use different tubes there, I'll need ones that use roughly the same plate voltage? Or am I misunderstanding the situation there . . .

I did look at the 5C5, actually, as a model. I don't need/want 2 channels, though, so I was thinking of using the extra preamp tube to build a couple extra gain stages. Is that worth doing on a relatively low-powered amp like this? Or will I have unusable amounts of gain if I do?

Thanks for your tips! I keep realizing the number of holes in my knowledge about tube circuit design (okay, about the entire world of electricity). . . but slowly I'm filling them in.

--Colin

[Bruce / Mission Amps]

Thanks, Bruce. I guess I'll keep it easy and use the 12V tubes. The output tubes were originally 12A6s. Would it be worth using those for a guitar amp? What would be a useful replacement? The OPT has a pair of taps to feed current to the plates on those, and I don't know the voltage (250V would be the max on those, right?)-- so I am guessing if I use different tubes there, I'll need ones that use roughly the same plate voltage? Or am I misunderstanding the situation there . . .

I did look at the 5C5, actually, as a model. I don't need/want 2 channels, though, so I was thinking of using the extra preamp tube to build a couple extra gain stages. Is that worth doing on a relatively low-powered amp like this? Or will I have unusable amounts of gain if I do?

Thanks for your tips! I keep realizing the number of holes in my knowledge about tube circuit design (okay, about the entire world of electricity). . . but slowly I'm filling them in.

--ColinI wouldn't get too carried away with unneeded or possibly complex gain stages and all that until you can decide if you can really do this.

I'd build a lower power version of a 5B4, 5C5 or 5C3 (etc) now and try and rewire the power tube sockets for a pair of 6V6s, but run their two filament leads in series at 12v. You can parallel the two gain stages for a little 3dB boost if you want but you'll probably like it stock for while, I think.
The power tranny you have is heavy enough for the two Push Pull 12A6s to be around 10-12 watts or so in soft Class A to Class Ab so I think a pair of 6V6s in class AB1 at around 400vdc ought to be fine for now. You might need to use a 360 ohm or higher cathode biasing resistor for the two 6V6 power tubes.
I'd be surprised if you didn't get and easy 12 watts RMS out of it.
Your OT might be around 6K to 8K now so it should be OK to.
Later you can redo it with a tweed 5F6 Bassman type topology and keep the 6V6s.

[G-hoppa]

Great tips-- thanks. Time to figure out a plan of attack. Tweak that 5C5 schematic first, I guess. I'll post back with news . . . and more questions, no doubt. Thanks for your quick and thorough responses!

[Bruce / Mission Amps]

Great tips-- thanks. Time to figure out a plan of attack. Tweak that 5C5 schematic first, I guess. I'll post back with news . . . and more questions, no doubt. Thanks for your quick and thorough responses!NO problem. I enjoy it or I wouldn't be here.

[G-hoppa]

Since I'm going to be building a 5C5, which calls for a different rectifier than this originally was built with, I'm assuming I'll need to use the rectifier the 5C5 uses (5U4 or soomething). Can I assume this will not be a problem when paired with a PT that was originally feeding current to a 5Y3? Is the only thing to worry about for rectifier choice the tubes on the output end-- the destination for the B+?

Thanks

[PRNDL]

One of my first amp builds was converting a Heathkit A7E into a guitar amp. The 12A6's have about 7-10 watts.
Heathkits were kit amps, so the builds varied greatly. Mine was very neat and had excellent coupling caps.

There's a bunch of sound clips on my web page
http://www.naturdoctor.com/Chapters/Amps/Retro.html

I put the A7E into a tweed head
http://www.naturdoctor.com/Chapters/Amps/Photos/Retro.JPG

[G-hoppa]

PRNDL-- that does sound great. Beautiful tweed cabinet, too! So the only mods you made were to the tone stack? Did you keep the two-channel configuration?

I've been planning to put a pair of 6V6s in mine, but you are maybe changing my mind . . .

[PRNDL]

I changed the tone stack and made it into a two channel amp using the push-pull pot (I put in a new one). The second channel has a bit more gain. Also, the input was changed by removing the RIAA circuit and 68K/1M pots on the input jack, which is now on the front panel.

There's a difference in plate resistance between the 6V6 (5K) and 12A6 (8K), plus an increase in wattage, which may be problematic.

12A6's are pretty cool tubes - inexpensive, and usually built to military specs.

[G-hoppa]

PRNDL-- what components on the A-7 constitute the RIAA ciruit? Beginner question, I know.

I think I'm going to backtrack on my earlier plan to create a Fender 5C5 on this chassis, and rebuild the A-7 to original circuits (except the few changes you've noted). I think it might be just as instructive for me, and less likely to end up a mess. Besides, I was impressed witht the range of useful tones you were able to get from the original tube complement.

Thanks

[loudthud]

Great looking head box PRNDL !!! You don't see many tweed cabinets with that big radius on the corners.

I found a schematic if anybody is interested:

http://www.audiophool.cjb.net/Schem_A/Heath_A7.gif

There might be a problem with the 5U4 drawing 3 amps on the heater supply, the 5Y3 only needed 2. You could try a GZ34 if a 5Y3 won't do the job but I think you should try the 5Y3 first.

[G-hoppa]

Aha-- I wondered about the rectifier. Thanks Loudthud. Yeah-- I will for now stick to the original A-7E plan. 5Y3 it is. Just ordered the tubes-- couldn't find anyone who would match a pair of the 12A6 power tubes, though.

In the meantime, anyone out there have a nice photo of a ground buss in place? My last project was a grounding nightmare, and since I have this chassis stripped to the sockets, I might as well try another approach. I know opinions are strong on grounding-- star vs., well, whatever else people do. I'm intrigued by the single copper wire running the length of the chassis--sounds clean and easy to understand (important to me!). But I want to see what it actually looks like in place, and how/where people have mounted it.

Thanks

[loudthud]

One thing that frequently causes problems with grounding is the extra chassis connections at the input and output jacks, and possibly at the filter cap can. Without knowing these things, it would hard to predict what will work. It would be best to have individual caps in the power supply and jacks that don't automatically make connection to the chassis. You are wise to try to plan this out before construction. I can only assume the original was built on terminal strips. Can you post a picture of the inside of the amp?

[G-hoppa]

Well, I didn't take pics before I gutted it. There are some rust spots inside, and I figured I'd strip it all out and treat those, since the hammered finish on the outside is so nice. But I removed literally everything. Pics won't help.

It did have terminal strips, which I kept.
I'll be using new filter caps, so the can will be there but not connected to anything. It is mounted on an insulating wafer.
I'll use self-grounding input/output jacks that are insulated from the chassis, so that should solve the problems you mention.

one original input jack was mounted to ground to the chassis, and had a soldering connection for grounding that circuit to the chassis right there. Am I right to think that this point is a common place to ground a buss to the chassis? or should the point of contact for that buss be down by the power cable's 3rd wire grounding point? (I'm assuming that the point of the buss is to have only one contact with the chassis, so all ground traffic flows there, like a stretched out star grounding scheme-- am I right?)

Thanks for your help

[J Martin]

I own one of those amps and I just restored one for a friend. Those amps don't need much of any mods. I wouldn't waste time trying to convert it to a Fender, you can buy Fenders off the shelf. My procedure was to convert the single triode (12SK7?) to a dual triode 12SL7 and make the new triode guitar friendly. It sounded really good. I did all the caps as well. That alone increased the volume quite a bit.

I just bought a whole box of 12A6 tubes at a hamfest, so I will be doing some experimenting. My Heath, and my friends's both had 12V6 as the power tube. Later versions, I guess.

[G-hoppa]

Well, I can't get a Fender 5C5 plus hours of soldering fun and learning off the shelf for the little I spent on this Heathkit. I was looking forward to the experience more than anything else.
That said, I am definitely coming around to your point of view -- listening to PRNDL's recordings helped me decide to bring this back to life as an essentially stock A7 with a few slight mods, to the input circuit and the tone. I got the manual for this thing in the mail today-- so cool. Even tells exactly how to disable the RIAA equalizer to get a flat (and higher gain) signal off the 2nd input.

Funny you mention the 12V6s--there's a note by the original owner of the manual that indicates the only change necessary to use 12V6s instead of 12A6s is to swap the 2W 470ohm cathode biasing resistor to an 800ohm, or throw a 330ohm in series with the original. (am I right about that resistor's purpose?)

What do you think the tone/wattage difference would be between the two power tubes?

[loudthud]

I made some changes to the schematic with Windows Paint. Changed a couple of things like rectifier connection to make it GZ34 compatable. This thing will have a heck of a lot of gain. Rearranged the ground per Kevin O'Connor. Added a fuse at the last minute.

[PRNDL]

PRNDL-- what components on the A-7 constitute the RIAA ciruit?
All the RC's after the first gain stage, except for the coupling cap.

I thought the circuit looked more like a 5e3, with the cathodyne phase inverter.

Also realize that the speaker (an EV SRO Alnico) is a big part of that tone.

[PRNDL]

This thing will have a heck of a lot of gain.

That's true ... way too much gain for input 1 and not enough for input 2.

What I did was use only one input and rewire it to switch between the two gain stages, one with half the gain from the 470K/470K resistors.

I also clipped the 200MMF cap on the PI grid, which is there to limit HF oscillation. There wasn't any perceptible difference, but I didn't care to solder it back in.

I may have changed the feedback to the 4 ohm tap to give it a bit more grit, but NFB makes amps have better cleans.

As for matching 12A6's ... back then I didn't know enough to ask for that or check the bias. I may have been lucky, but 12A6's in metal cans are rugged!

12A6's have the same pin out as 6V6/6L6, so you can use any of the standard bias meters to match them.

[G-hoppa]

loudthud-- I see you've routed the filter cap grounds to the cathode grounds of the tubes they feed--what is the rationale behind this approach? Controlling the current path to avoid . . . what exactly? My grasp of grounding concepts is shaky.

[loudthud]

There are really two reasons to separate the grounds. First, the ground of the first filter cap after the rectifier is the noisiest ground in the whole amp. It's the buzzy kind of noise at twice the line frequency. A push pull amplifier stage can reject quite a bit of this but a single ended stage can't. By separating the grounds, we make sure that the buzz doesn't get on the ground side of the down-stream preamp filter caps (relative to the ground at the preamp stages). If there is noise on the ground side of a filter cap, the cap conducts it to the B+ side with almost no attenutaion.

Secondly, there are small signal currents that must flow from B+ to ground (through the filter cap) at each preamp stage. Having several stages on one filter cap means there can be some feedback from one preamp stage back to the one before it. This can cause an oscillation if the gain is high enough. So it's best to keep the path for these currents as short as possible. Sometimes it becomes necessary to add another RC filter to isolate two preamp stages.

If the high gain is too problematic, try substituting a 12SN7 (mu=20) for the 12SL7 (mu=70). Other than that, removing one stage or adding attenuation between stages should control the gain to workable levels.

[G-hoppa]

What I did was use only one input and rewire it to switch between the two gain stages, one with half the gain from the 470K/470K resistors.

PRNDL-- can you break it down a little for me-- what does this look like? You mentioned a push-pull pot for the high/low gain selection. So, would this bypass the first triode in the 12SL7? And the pot's output would still go to the 12SQ7?

[PRNDL]

PRNDL-- can you break it down a little for me-- what does this look like? You mentioned a push-pull pot for the high/low gain selection. So, would this bypass the first triode in the 12SL7? And the pot's output would still go to the 12SQ7?

Only one of those two gain stages are needed, which means there is a choice:

- Matchless Spitfire left the 1/2 a 12AX7 unused
- Marshall wired each side to a pair of inputs - high and low, which many jump in parallel
- 18 Watt Light II wires to a pair of inputs - single and parallel
- Fender Pro Junior is similar to a Spitfire with the extra gain and a divider network
- Some amps switch in an extra gain stage for distortion

There's no room on the front panel to put two input jacks or an additional volume/tone for the distortion channel. It was a bit complicated, but I used a DPST push/pull pot to switch between one of those gain stages. The idea was to add a little bit of boost. Initially I used the existing pot, but the slider switch wasn't that great, so I replaced it.

As you can tell from the sound clips, the amp on 10 gets a decent amount of distortion. You may want to try having both gain stages, which will give tons more gain for metal. The risk is too much gain causing a nasty buzz.

[G-hoppa]

I made some changes to the schematic with Windows Paint.

loudthud-- belated thanks for this. And for the explanation of the grounding scheme. this is invaluable information for me at my basic level of understanding.
PRNDL--thanks to you too-- I will cook up something along the lines of the Spitfire, I think. No need to aim for metal distortion.
Now I'm just waiting for some parts to get started . . .

[CitizenCain]

Say PRNDL, what values did you use to tune the tone stack for guitar? I have an old Heathkit AA-23 that I'm converting for guitar use. The tone stack is contained in small plastic-encased module. Itwould be hard to change any component values in it. If the stock onesa re close enough, I'll just use it as is.

[PRNDL]

what values did you use to tune the tone stack for guitar?
I used the tone stack calculator to match the curves to the existing pots.

These are the values for the Heathkit
Resistors: 100K, 33K, 100K
Caps: .002, .02, 200p, .002
Pots: 1M-A, 500K-A

These are the standard values for a new build
Resistors: 68K, 56K, 220K
Caps: 500p, .0047, 500p, .001
Pots: 500K-A, 500K-A

The main issue I found was designing the tone stack with values of caps that are readily available, so that you don't have to buy 2 to get the right value.

The problem with the James circuit board is you can't tell what the values are inside. The simplest method is to remove it, and use new caps and resistors.

[CitizenCain]

Those are interesting values. I ran them through the tone stack calc, neat curves. Yep, I think just creating a tone stack would be simpler than trying to modify the little circuit card these amps use.

[G-hoppa]

I made some changes to the schematic with Windows Paint. Changed a couple of things like rectifier connection to make it GZ34 compatable. This thing will have a heck of a lot of gain. Rearranged the ground per Kevin O'Connor. Added a fuse at the last minute.

Loudthud-- I notice on this you've changed the B+ for the 12SL7 from 180V to 330V. Is this the mod you made to enable the use of a GZ34? Since I'm going to keep the 5Y3, can I simply use the original 100K resistor there at the last filter cap?

What would such a large voltage difference have done to the gain and tone of the 12SL7?

[G-hoppa]

okay-- here's the schematic I'm going to try-- please take a look and let me know what you think. I've kept Loudthud's grounding scheme, borrowed the tone stack values PRNDL mentioned, and bypassed the first (unneeded) gain stage. Not sure about that first gain stage, though . . .

Let's see if I can figure out how to upload an image.

[loudthud]

Loudthud-- I notice on this you've changed the B+ for the 12SL7 from 180V to 330V. Is this the mod you made to enable the use of a GZ34? Since I'm going to keep the 5Y3, can I simply use the original 100K resistor there at the last filter cap?

What would such a large voltage difference have done to the gain and tone of the 12SL7?
The original amp was intended to amplify a phono cartridge. Those signals are much smaller than a guitar signal. A higher B+ will increase headroom. The lower B+ would work good for a harp amp though. For a GZ34 you just need to take the output from pin 8 instead of pin 2 like the original.

[G-hoppa]

Thanks to all who helped with suggestions. I finally got the Heath A-7E finished and played through it with the band last night-- better than I could've hoped. It's not just a working guitar amp-- it sounds fantastic! Crunchy, tight,--better than my current Peavey Classic 30. (granted, it has a smaller range of sounds to dial in than the Peavey, but I tend to play with a particular sound, and this Heath does it better) I have yet to build a head box for it, and one of the NOS tubes (6SL7GT) is microphonic, but all in all, an encouraging first build.

Now I need to dig into the old Bell hi-fi I have and create another. It's addictive.

-C

[CitizenCain]

Congrats! I'd love to hear how this turned out, I can sort of visualize it.

I ran into some stumbling blocks on my Heathkit conversion and some other things in life have forced me to put it on hold for the last month. Hope to get back to it in a week or so.