Allen 5F1+ (or something like it)

[munkeyboy]

I have a PA amp that I'm rebuilding.  It is a SE 6L6(GB) with a 6av6, 12ax7 and 5y3.   I found the Allen 5F1+ layout on this site after looking for a similar type of amp.   Thought this PA amp would be pretty good candidate for this Allen 5F1+ build.  However, I can't find much info or schematic on the Allen 5F1+, just the layout.  I have competed a first build, but am having some issues.   I think I saw mentioned that the 5F1+ had gone thru a few revisions and is Allen is no longer producing it.  Does anyone have more info or know if there were known issues with this circuit?  Any other good SE circuits with the extra gain stage?

[sluckey]

What issues do you have?

Should be pretty easy to draw a schematic from that layout. Might want to start with the schematic for a real 5F1 and add the extra gain stage.

[munkeyboy]

What issues do you have?

Should be pretty easy to draw a schematic from that layout. Might want to start with the schematic for a real 5F1 and add the extra gain stage.

Two issues are probably my own fault.  I'll get to that after.   I really wondering if that Master volume is correct.   It seems to work until about half way, then it starts to drop volume again (but not all the way).

The other issues are probably my fault.  There seems to be higher than expected voltage across power cathode cap (~63v with MV turned up, ~27v MV turned down).  This exploded the first cap I had there.   I'm also having constant hum (power).  This is probably due to poor design on my part.  Gut shot posted here anyone could give me hints about what needs to move... probably filter caps too close to OT?  Chop-sticking changes nothing.

[munkeyboy]

note: i keep the original values for the power section (and power tube cathode) since I didn't know what PT the Allen amp has.   This one does have an odd 270k to ground (r19).

[sluckey]

The MV wiring on the layout is fine. Changing the MV should not change the cathode voltage at all. Something is not wired IAW the layout.

[Williamblake]

That bleeder might be replaced with a small outputtube since it would draw some 36mA.

/ What was i calculating?

[DummyLoad]

That bleeder might be replaced with a small outputtube since it would draw some 36mA.

huh?

270k bleeder is running with a 360V source so 360V / 270K = 1.33mA

285V source is drawing 2.27mA

360V source is drawing 6mA total which includes the 270K bleeder current and 285V source load current. so devices hanging off the 360V source are drawing (2.27mA  + 1.33mA) - 6mA = 1.97mA -

series strings current loads are calculated as 420V - 360V = 60V / 10K and 360V - 285V = 75V / 33K

--pete

[munkeyboy]

Man, I don't know why I didn't put together it was a bleeder.   I don't really need it since I discharge with my own little resistor/alligator clip every time I work on it.   That would give back some  current if needed, but doesn't seem like I do.

So in the Allen amp, what would be the benefit of having the extra 47uf filter cap and the choke?    Haven't seen that on other 5F1/5F2a builds, but again I don't know what the specs are on that allen PT.    Also note this PA amp (bell pacemaker 10) is using the lower output 6L6GB, so not quite the same as 6L6GC.  Another reason I've keep the power and cathode values the same.

[munkeyboy]

The MV wiring on the layout is fine. Changing the MV should not change the cathode voltage at all. Something is not wired IAW the layout.

Actually back to this issue before I derail the thread on the power supply.  I will go thru the layout a forth and fifth time, I'm sure I must have missed something.  However, if my layout is correct (or even if it isn't) what possible things might contribute to increased voltage across the power cathode?  I didn't see continuity issues with the tube socket.  Leaky coupling caps?

[shooter]

contribute to increased voltage across the power cathode

what would ohm say 
if B+ is steady, if Vk goes up then current went down.  does MV "adjust" the Vk voltage? 

other things;
Rk got stressed and is no longer the value you started with, same might be said of tube, B+ changed while you weren't looking, OT impedance, speaker impedance

all that ASSUMING it's wired correct and the "design" has been proven before you had issues.

[sluckey]

Need two sets of voltage readings. For set one... With MV turned all the way DOWN, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

Now for set two... With MV turned all the way UP, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

No signal into the amp while measuring voltages. But connect a speaker and listen for bad sounds. Report back.

[DummyLoad]

TP25 specs are 620VCT @ 150mA

http://www.allenamps.com/trans.html

the added filtering is to lower PS buzz due to ripple - probably to allow use of 12" drivers over an 8" or 10". class A (SE) output stages will amplify PS ripple. push-pull output stages tend to cancel PS ripple.

i sketched a schema from the layout provided in reply #1. hope d. allen is not offended.

--pete

EDIT: fixed PT part model # in schemas.

[munkeyboy]

Need two sets of voltage readings. For set one... With MV turned all the way DOWN, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

Now for set two... With MV turned all the way UP, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

No signal into the amp while measuring voltages. But connect a speaker and listen for bad sounds. Report back.

With MV down:
3 = 390v
4 = 405v
5 = .3 mv
8 = 27.5v

With MV up:
3 = 265v
4 = 296v
5 = 68v
8 = 65.3v

[munkeyboy]

TP25 specs are 620VCT @ 150mA

http://www.allenamps.com/trans.html

the added filtering is to lower PS buzz due to ripple - probably to allow use of 12" drivers over an 8" or 10". class A (SE) output stages will amplify PS ripple. push-pull output stages tend to cancel PS ripple.

i sketched a schema from the layout provided in reply #1. hope d. allen is not offended.

--pete

Wow you are quick.   I need to learn that program.  Which one is it?

[DummyLoad]

express SCH/PCB - download the PCB application - schema capture is included.

--pete

https://www.expresspcb.com/   - download classic.

[munkeyboy]

Need two sets of voltage readings. For set one... With MV turned all the way DOWN, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

Now for set two... With MV turned all the way UP, measure voltages on pins 3, 4, 5, and 8 of the 6L6.

No signal into the amp while measuring voltages. But connect a speaker and listen for bad sounds. Report back.

With MV down:
3 = 390v
4 = 405v
5 = .3 mv
8 = 27.5v

With MV up:
3 = 265v
4 = 296v
5 = 68v
8 = 65.3v

Besides the ever present loud 120hz hum, moving the MV does not really produce any bad sounds except some crackle (due to voltage on the pot i guess) and the change in volume/static as expected.  But if there was input, it would behave as I explained before where at about halfway the volume starts to drop off again.

[sluckey]

5 = 68v

This is what is causing the cathode voltage to go high!!! Pin 5 should always be ZERO volts. Disconnect one end of that disc cap on the board that's connected to V2 pin 6. Your photo shows a red gator clip lead attached to it. Recheck voltages on the 6L6. Better now? If so, replace that disc cap.

[shooter]

Pete you are the GEEK 

I hate ts'in from layouts.

[DummyLoad]

Pete you are the GEEK 

I hate ts'in from layouts.

thnx. i like these kinds of puzzles. some of the old PA amps richard and i had acquired over the last +/-15 yrs. had no schematics, so we'd make our own. most times we end up gutting most all of the ckts., however, we'd have reference for rebuilding the PS and output stage. after some repetition, transposing from built to schematic becomes as trivial as a card game.


--pete

[DummyLoad]

per your request.

--pete

[munkeyboy]

5 = 68v

This is what is causing the cathode voltage to go high!!! Pin 5 should always be ZERO volts. Disconnect one end of that disc cap on the board that's connected to V2 pin 6. Your photo shows a red gator clip lead attached to it. Recheck voltages on the 6L6. Better now? If so, replace that disc cap.

Yep that was it. I have another disc cap on hand and that worked.  Thought those old disc caps were indestructible.  I have new Mallory caps on order anyways, was just using these until my order came in.

Sounds pretty good and super loud, but the breakup is a bit harsh.  However I can tune that later (and replacing those disc caps should help).

There is still pretty bad hum in this guy.  I have one ground for preamp grounded at the input, one ground for the power section and one for the wall plug.   I'm not 100% sure, but I think it is not ground loop issue.    Can you see any issues with my wire routing or filter placement (too close to OT?)?

[munkeyboy]

per your request.

--pete

Thanks Pete!

[DummyLoad]

you really want the extra filtering as shown in the allen 5F1+ plan. with a class A power amp, as much as you dare add - a Pi filter for entire amp would kill nearly all of the PS buzz. try disconnecting or using a larger NFB resistor to help with the harsh OD character.


--pete

[munkeyboy]

Ok, guessing I should pickup a choke or would a dropping resistor work alright?  I already have the NFB disconnected btw.

[munkeyboy]

Well right after the last post, my parts were delivered on my front door.  A few days sooner than i was expecting.   

The new coupling caps help the OD a little.  However that first gain (input 1) is still pretty harsh if dimed, could just be the nature of adding this extra gain.  I don't know if harsh is the right word, but it's not smooth.  I'll try some different 6av6's to see if that helps.  Bypassing that first gain (using input 2) sounds great.  Not too much breakup however.  NFB doesn't add too much to it either.   

Kind-of hard to appreciate the amp until i can get rid of that loud hum though.  Maybe I'll go ahead an order a 125C3A choke with an extra filter cap.  Wish there was an amazon prime for electronics... or a physical store nearby.  I think I spend more on shipping than I do parts.   I've tried to order ahead, but it's the little one off items that get you.

[shooter]

until i can get rid of that loud hum though

for fun n giggles could you measure the AC at the DC power rail taps, my kinda sorta good is <2vAC at the power tube tap, < .2vAC at the preamp taps.

also didn't read the whole thread, did you pull preamp tubes to see if hummm changed?

[munkeyboy]

until i can get rid of that loud hum though

for fun n giggles could you measure the AC at the DC power rail taps, my kinda sorta good is <2vAC at the power tube tap, < .2vAC at the preamp taps.

also didn't read the whole thread, did you pull preamp tubes to see if hummm changed?

I'll have to do this measurement later.  But to answer the last question, yes, I had removed the preamp tubes and hum is still present.  Also doesnt change with volume.

[DummyLoad]

CRC will work well, CLC a magnitude better.

might try 5Y3 > 33uF > 100R > 33uF > OT  - or -  5Y3 > 33uF > choke > 33uF > OT.

large fender chokes are priced reasonable and would work in this application. 4H 90mA - DC resistance is about 100R and are a 500V part. 

--pete

[shooter]

Was looking over DL's F1+, which caused me to go read when music is sounding soooo good 

from Mr. Wizard;

" The suppressor grid should therefore always be connected to the cathode (unless you're doing something really clever / peculiar). "

Is it clever or does it really matter or just a typo 

[kagliostro]

Ciao Shooter

If I've understand correctly what you mean ...

pin #1 is not G3, it is connected to nothing on 6L6  (on KT88 and 6550 pin #1 is connected to the (metal) base shell)

also the tube is a beam tetrode and, yes, beams are connected to the cathode, pin #8

Franco

[DummyLoad]

" The suppressor grid should therefore always be connected to the cathode (unless you're doing something really clever / peculiar). "

no sir, what allen did is attach suppressor to a 3rd ground point that was convenient. EL34 suppressor connection (pin 1) in schema i drew is not a typo - it was traced off the layout plan taken from reply #1.

in fixed bias, the cathode is tied to ground and then g3 is connected to the cathode as a matter of convenience. if you scrutinize EL34 spec sheets they all state Vg3 is tied to 0V which is ground potential.

with respect to the cathode V if in auto-bias if Vg3 is at cathode potential, as far as the overall system is concerned, when g3 is at same potential as the cathode it is at essentially 0V.

so, it seems that d. allen took the literal from spec sheets and wired g3 at ground potential.

--pete

[munkeyboy]

for fun n giggles could you measure the AC at the DC power rail taps, my kinda sorta good is <2vAC at the power tube tap, < .2vAC at the preamp taps.

So how does one measure AC at the DC without the multimeter jumping all over the place?

[kagliostro]

no sir, what allen did is attach suppressor to a 3rd ground point that was convenient. EL34 suppressor connection (pin 1) in schema i drew is not a typo - it was traced off the layout plan taken from reply #1.

That is exact, my fault, I was considering only the 6L6 option

Franco

[shooter]

set you meter to Volts AC,  measure VAC, the meter can't measure the AC riding on the DC if the meter is set to DC

Thanks, guessing the difference wasn't much fixing it to cathode or ground

[munkeyboy]

set you meter to Volts AC,  measure VAC, the meter can't measure the AC riding on the DC if the meter is set to DC

Haha, yes.  But on the Volts AC setting, my digital meter jumps around if trying to measure AC when DC is present.  shows anywhere between 1vac to 130vac (guessing here as it's hard to read when i flashes so quick).

[shooter]

then you really need a new quality meter for your resolution of 2020 

[munkeyboy]

then you really need a new quality meter for your resolution of 2020 

This is true.  But it is either that or a scope.

[munkeyboy]

CRC will work well, CLC a magnitude better.

might try 5Y3 > 33uF > 100R > 33uF > OT  - or -  5Y3 > 33uF > choke > 33uF > OT.

large fender chokes are priced reasonable and would work in this application. 4H 90mA - DC resistance is about 100R and are a 500V part. 

--pete

So a lot of the hum was due to a bad filter cap(s).  I thought the ones I had on there were recent caps from a failed build, but nope.   Any how at the same time I added the extra filter cap.  Bought a choke but haven't installed it yet as I'd like to circuit a clean as possible before I commit to drilling new holes.   Instead I put a 100R between the first two caps.   I already had new 47uf, so using them instead of 33uf.  This is what i have: 5Y3 > 47uF > 100R > 47uF > 10k > 10uF > 33k> 10uF.  Every cap is brand new.


Sounds much better, but still a lot of noise. However, upon turning MV down, hum gets progressively louder.  Not sure if it's two different sources of hum.   There is also a bit of buzz and/or 60hz hum coming from the preamp.  Some might be from a noisy 6av6 (already thru one away).   But there is probably some from lead dress.  The lead from the 2nd input switch to the 680k/220k has to be right next to the chassis or it produces hum.  Should this be shielded or is that overkill (it's the brown wire running along the chassis from the input)?

[shooter]

or is that overkill

kill it with a bazooka, killin hum is legal in all States 

[munkeyboy]

So I did that plus the leads to the coupling caps and the leads between volume and tone since they cross the MV pot.  Such a pain to work with shielded wire, I have about 8 shielded runs now.  I also re-worked the ground so the bus wire is not soldered to the back of the pots to make sure i have minimal ground points.   All that helped cut out most buzz/static so yay!     But I still had to route the two tone leads away from each other for some reason even tho they are shielded.  I wish I could put the tone next to the vol, but the front panel is not laid out that way (mic [vol] > phono [MV] > acoustic control [tone] > power). 

However, I still have that hum from the PS.  Removing NFB wire just makes it louder.  Hum is still there when removing both preamp tubes and I've tried different 6L6s and 5y3s with no change.  I also have static and/or sudden change on the pots when at either end of the wiper, but nothing bad along the travel.  I must have some sort of short to ground that i just can't find.  First I'm going to put back the humdinger pot elevated to the cathode to see if that helps hum instead of 2x 100ohm to ground.  Not sure if the addition of the choke will help either.

BTW.  Reading the original schematic for the PA, the plate-to-cathode voltage is about 374 and the cathode drop is 26. This is somewhat inline with what i'm reading now, tho mine plate-to-cathode is about 355.   The original amp has a 6L6GB (not GC).  That means it was running at about 138% plate distribution or 26w.  That pretty significant change from what a normal 6L6GB should run (19W).  It also means I'm currently running at 131% plate distribution or 25w.  Are my calculations wrong?  Was there a version of the 6L6GB in the 50's that is 25w not 19w?

[DummyLoad]

RCA datasheets state their part is 19W anode and 2.5W g2 - Tung-Sol states their part is 22W anode and 2.8W g2. if it's not red-plating, then it's going to be ok.

as for the hum, it seems as though you've hit a brick-wall with that layout and/or design.

Removing NFB wire just makes it louder.

at least you have the polarity of the NFB correct. ;-)

--pete

[munkeyboy]

RCA datasheets state their part is 19W anode and 2.5W g2 - Tung-Sol states their part is 22W anode and 2.8W g2. if it's not red-plating, then it's going to be ok.

as for the hum, it seems as though you've hit a brick-wall with that layout and/or design.

Removing NFB wire just makes it louder.

at least you have the polarity of the NFB correct. ;-)

--pete

Yep, I think it's time to scratch it.  Too bad sounds good otherwise.    I did put in that humdinger pot.  Set it to the mid point and turned on the amp.   Turning it to one extreme did nothing hum wise, to the other extreme it blew the fuse.

[sluckey]

I did put in that humdinger pot.  Set it to the mid point and turned on the amp.   Turning it to one extreme did nothing hum wise, to the other extreme it blew the fuse.

This sounds like one side of the filament winding is connected to chassis ground. Check that and remove from chassis. Then try the humdinger again.

[munkeyboy]

Not sure what it was.  Couldn't find any filament connected to chassis.   It was a 1a in there before, but replaced with a 2a.  No issue this time. 

FYI the choke helped remove a lot of hum.  It still has a little hum with MV turned up and I could live with it if I didn't use the MV.   Since I'm close, I may not give up on this layout, just take a break for a few days. 

[pdf64]

RCA datasheets state their part is 19W anode and 2.5W g2 - Tung-Sol states their part is 22W anode and 2.8W g2.

Apologies for bringing this up, but as this thread was referred to in another thread, I happened to notice this somewhat after the event :)
The RCA and TungSol 6L6GB are exactly the same valve, it's just that the RCA info was published in 1957, prior to the design maximum rating system being adopted in North America in 1958, so the ratings are, as noted on them, under the design centre system.
Whereas the TungSol info was published in 1961, and so as noted on the ratings header, they're under the design maximum system.

6L6, G, GA and GB all have a 19W  design centre, and a 22W design maximum, anode dissipation limit.

The 6L6 will have an absolute maximum rating somewhat above the design maximum.


I think that redplating will manifest somewhat above the absolute maximum limit.

[passaloutre]

Having just finished a 5F1 with 6L6, I find it really hard to imagine what kind of fire breather it would be with an additional gain stage!