Heathkit A-6A Tubes

[Gene Barnett]

Can someone help me? I have a Heathkit A-6A (trying to get working). I have a schematic but It doesn't indicate WHAT the back center position tube is supposed to be. (The front center position is a 6 SN7). Would the back be another 6 SN7? Somebody threw a 5Y3 tube in that position but it keeps heating up way too much. Does somebody know? Thanks so much!

 

[Dave451]

Welcome to AK! The A-6A schematic shows a 5U4G rectifier. The 5Y3 is a considerably lower rated rectifier, so if that's what's in the rectifier socket, it's probably way overloaded and would likely run quite hot. Can't find any other manuals showing tube location. Nothing marked on the chassis? A couple of pics of the top and bottom (cover removed) would help.

 

[century tek]

Hello! Yes pics please! The bottom side will definitely point to which tube socket needs to have the 5Y3 / 5U4 tube.

 

[Gene Barnett]

Welcome to AK! The A-6A schematic shows a 5U4G rectifier. The 5Y3 is a considerably lower rated rectifier, so if that's what's in the rectifier socket, it's probably way overloaded and would likely run quite hot. Can't find any other manuals showing tube location. Nothing marked on the chassis? A couple of pics of the top and bottom (cover removed) would help.

THANKS! You guys are the bomb! Here's a pic of the bottom. The socket at the top Center is the tube in question. The wires connected to the socket are all coming from one of the two transformers. Hope this helps.

 

[century tek]

Yeah, the socket at the top center is the 5Y3 / 5U4 rectifier tube. The big black transformer is the power transformer which takes in the AC wall voltage and converts it to various voltages needed through out the amp but the amp cannot work on AC so it needs to be converted to DC so this is what the rectifier tube does.

The 6SN7 would be your front center position.

Now here's the question. Now that you know what tube goes where, if the rectifier tube's plates (the outermost metal plates inside the tube) are glowing red, then it is doing what's known as red plating and that means you have a shorted filter capacitor. Looks like you have two of them which are the two cylindrical round things sticking up on the top side of the chassis. In your pic showing the underneath, the shorted one would most likely be the one on the left which has the wire connected from pin #8 of the rectifier tube.

If they are original, both of those filter capacitors should be replaced anyway. It looks like it has been updated with newer coupling capacitors so you should be fine in that department.

Let's see a top, front and rear view.

 

[Dave451]

Yep; concur. Those two multi-section 'can' electrolytic caps are probably bad (at least one bad section, I bet) and causing excessive current draw. Is the 5Y3 red plating? Once the caps are straightened out, I'd get a 5U4G to replace the 5Y3. The 5Y3 is about half the current rating of the 5U4G and max voltage is about 100 V less for the 5Y3, if I recall correctly. With two 6L6 output tubes, you'll need the capacity.

Also, it would also be desirable to cut the lead length down on those new coupling capacitors (mainly the big orange drop--I think it's 0.1 uF) to the minimum required to avoid noise, oscillation, etc. Also, be sure you have the right fuse installed in the holder? Schematic doesn't show one (!), but one is installed. If there's no value shown on the chassis (or your schematic), something like 2-3 amp slow blow is what you want.

Not sure what your experience/comfort level is for working on tube gear--beware very high voltages that can give a bad shock or even kill you if you get across them. If you don't have this level of experience, better to have it done by someone who does. Best of luck with the amp--let us know what you find out and do!

 

[century tek]

If it uses 6L6 tubes, then yeah, I would also concur to use a 5U4 tube.

 

[Dave451]

Just going by the schematic

 

[Gene Barnett]

Yep; concur. Those two multi-section 'can' electrolytic caps are probably bad (at least one bad section, I bet) and causing excessive current draw. Is the 5Y3 red plating? Once the caps are straightened out, I'd get a 5U4G to replace the 5Y3. The 5Y3 is about half the current rating of the 5U4G and max voltage is about 100 V less for the 5Y3, if I recall correctly. With two 6L6 output tubes, you'll need the capacity.

Also, it would also be desirable to cut the lead length down on those new coupling capacitors (mainly the big orange drop--I think it's 0.1 uF) to the minimum required to avoid noise, oscillation, etc. Also, be sure you have the right fuse installed in the holder? Schematic doesn't show one (!), but one is installed. If there's no value shown on the chassis (or your schematic), something like 2-3 amp slow blow is what you want.

Not sure what your experience/comfort level is for working on tube gear--beware very high voltages that can give a bad shock or even kill you if you get across them. If you don't have this level of experience, better to have it done by someone who does. Best of luck with the amp--let us know what you find out and do!

Thanks so much again!
Yes, have experience working on this stuff, although not with an infinitely technical know how, thanks for the warning there. Grabbing a 5U4G tube, then will, install, check voltages, test or replace the filter caps stated, (thanks for noting where they are located), then shorten the leads on the other cap mentioned. Thanks again fellas! I will post the results if I can get this thing up & running again. It was a garage sale score which is always immensely fun!

 

[Gene Barnett]

So, I've got the 5U4G tube but, yes, there is "red plating" & the multi section filter Caps are bad & can't get replacements. Question: Is there a common "input" to the Multi section filter Capacitor-three caps (if that's correct to state it)? Thusly, would I connect one side (negative side?) of a 10, 20 & 30 mfd cap and solder the (positive/other?) side of each of the three caps to each "output"? There is a square symbol (30 mfd), a triangle Symbol (10 mfd) and the third "-" symbol (20 mfd). Would this be how to replace those filter caps?

 

[Dave451]

Gene,
You will need to remove each section of the bad multi-cap from the circuit and replace it with an equivalent capacitance and (important!) maximum working voltage (these are stamped on the multi-section cap). You can see if you can find a replacement can with the same values (or close for the capacitance; a somewhat higher working voltage is acceptable) and remove the old cans and put the new ones in place (replace both using the same mounting method as the original caps). Mark what symbol/value each wire connected to and re-connect them to the equivalent terminals on the new cap.

A second way is to purchase new axial or radial-lead single value caps matching the capacitance and voltage of each section of the multis and install them under the chassis in the appropriate parts of the circuit, leaving the disconnected cans in place for the 'look'. Disconnect all the leads from the old multi's and install the new caps in the appropriate locations in the circuit. Use terminal strips mounted to the screws coming through under the chassis or drill new mounting holes if you need to. NOTE: Do NOT just solder replacement single value caps to the terminals of the old/bad multis! This leaves potentially shorted/bad sections in the circuit (sounds like you have a short if tubes are red plating, but why isn't the fuse blowing?) and, if a section is good, you are essentially paralleling a new/good cap with the old section and DOUBLING the capacitance value.

Also, which tube/tubes are red-plating? What's the value of the fuse in the holder?

To get replacement cans, there are at least two options: 1) To to Antique Electronic Supply website and see if they have something close or 2) Send the exact specifications (capacitance values and working voltages by section and physical measurements) to Hayseed Hamfest and they will build you replacements. Either way, the new cans will be something like $30-40 each. (I have no affiliation with either vendor). There are other vendors that others may suggest. This may the the simplest way to replace the cans, but not the cheapest.

You can also cut apart the old caps, remove the guts and install new single-value caps to replace each section and reassemble and install. This is an involved process (called 're-stuffing') you can search for and see some good threads in the Tube Audio forum. It's a lot of work.

Dave

PS EDIT: changed "halving" to "doubling". My bad!

 

[Gene Barnett]

Gene,
You will need to remove each section of the bad multi-cap from the circuit and replace it with an equivalent capacitance and (important!) maximum working voltage (these are stamped on the multi-section cap). You can see if you can find a replacement can with the same values (or close for the capacitance; a somewhat higher working voltage is acceptable) and remove the old cans and put the new ones in place (replace both using the same mounting method as the original caps). Mark what symbol/value each wire connected to and re-connect them to the equivalent terminals on the new cap.

A second way is to purchase new axial or radial-lead single value caps matching the capacitance and voltage of each section of the multis and install them under the chassis in the appropriate parts of the circuit, leaving the disconnected cans in place for the 'look'. Disconnect all the leads from the old multi's and install the new caps in the appropriate locations in the circuit. Use terminal strips mounted to the screws coming through under the chassis or drill new mounting holes if you need to. NOTE: Do NOT just solder replacement single value caps to the terminals of the old/bad multis! This leaves potentially shorted/bad sections in the circuit (sounds like you have a short if tubes are red plating, but why isn't the fuse blowing?) and, if a section is good, you are essentially paralleling a new/good cap with the old section and DOUBLING the capacitance value.

Also, which tube/tubes are red-plating? What's the value of the fuse in the holder?

To get replacement cans, there are at least two options: 1) To to Antique Electronic Supply website and see if they have something close or 2) Send the exact specifications (capacitance values and working voltages by section and physical measurements) to Hayseed Hamfest and they will build you replacements. Either way, the new cans will be something like $30-40 each. (I have no affiliation with either vendor). There are other vendors that others may suggest. This may the the simplest way to replace the cans, but not the cheapest.

You can also cut apart the old caps, remove the guts and install new single-value caps to replace each section and reassemble and install. This is an involved process (called 're-stuffing') you can search for and see some good threads in the Tube Audio forum. It's a lot of work.

Dave

PS EDIT: changed "halving" to "doubling". My bad!

Thanks Dave, yeah, thought about restuffing the cans but it's far too laborious (filled with tar most likely). THANKS for the tips. No replacement cans available so...YES, will be replacing the cans with the right valued capacitors using terminal strips and connecting INDEPENDENTLY of the original multiband cans. Yep, leaving the cans in place. The caps came in last Thursday from Mouser so away I go this weekend. My amp tech guy said the thing should sound pretty good for a possible guitar amp. He said it could compare to earlier Vox AC series stuff, we'll see. If not, hopefully a nice mono block for some audio application. Appreciate the coaching!! Cheers!

 

[Dave451]

Good luck with it, Gene. Let us know how it goes!

 

[Gene Barnett]

Success. Filter caps replaced (with individual caps) and the amp works. Thanks for the great advice guys! SO, it has a small hum (maybe standard on these type amps?), like a 60 cycle thing. I will have to tech that out, possibly a cap that is backwards, possibly a hundred other things. I am thankful just to have it working generally. As far as it being a descent Guitar Amp, it seems to have a little redeeming value there but not enough to dedicate it to that. It takes a lot of "push" at the front end to have the guitar "react" with the amp to sound like a classic Fender etc. I'm guessing it would need more modding to make that endeavor work which will not be worth it in my case. It sounds good as a mono block which I believe is this amp's strength and value. Cheers!

 

[Dave451]

Good work! One trick would be to short the inputs and see if the 60 Hz goes away. If the hum follows the volume pot, then it's in the front end somewhere, maybe in the inputs. Be sure it's buttoned up (all covers in place) when you check it. Make sure that the shield/braid on the input coax coming from the input jacks is not grounded at both ends. It should be grounded at the end at the input jack itself. Sometimes you'll get a ground loop and 60 Hz hum if these shields are grounded at both ends.

An amp of this type is by nature stable and is going to be hard to drive into distortion or the kind of guitar sound you may be looking for, but as a mono block, not bad (as you say).

Electrolytic cap problems will cause 120 Hz buzz, instead of 60 Hz hum. If you had one of these backwards, it wouldn't last long before it heated up and failed. For film couplers, there really is no 'backwards' way to connect them.

Good luck with your efforts.
Dave

 

[knockbill]

I can't find a schem I can read for this amp... is it 4x6L6 and one 6SN7 mono amp?

 

[toxcrusadr]

https://elektrotanya.com/heathkit_a-6a_power_amplifier_sch.pdf/download.html

6SJ7x2, 6L6x2, 5U4 looks like.

 

[knockbill]

Thanks,,, I saw that one, but couldn't rotate or enlarge it... don't think its the right schem either,,, OPs has 6SN7,, and six tubes instead of 5?

 

[toxcrusadr]

You can enlarge that schematic by clicking on the almost invisible symbol at the top right. Then it lets you pan around with the cursor.

You're right, the OP's amp has 5 sockets in a row at the bottom and one up top.

 

[knockbill]

Yep,,, no need to open it!!! Like to know what its running... thread only mentions 6L6(s) and a 6SN7 "in the middle"...