parallel SE, Class A build, almost there

[centervolume]

hello, first timer here... thanks in advance for any sugguestions

I'm finishing up a custom circuit build (cathode bias, class A with 5881 and toggle to activate 2nd 5881 in case of emergency, with full assortment of treble bass mid controls [5f8-a tone stack] all in a tweed vibrolux box w/ alnico 10, 20 watt OT).

She's passing a nice full signal but with what sounds like a background 60 Hz hum in the power tube section (by background, I mean about 15-20% of the level of the signal)

I grounded all of the following to a single point:

1. filter caps
2. red/yellow PT center tap
3. power tube cathode bias assembly (this is the one I currently suspect as needing to be separated given its proximity to signal... does this sound like something I should ground to another location?)

I double checked my heater pairs, they hit the same pin #s on both 5881s and the same ones on the 2 ax/ay7s (I don't recall if they are supposed to alternate for example pins 4/5 on v1 go to pin 9 on v2 or if pin 4/5 on v1 should go to pin 4/5 on v2 etc)

I don't have a choke installed.

The heater has a center tap (green yellow) that I soldered to the inside wall of the chassis with my 80 watt solder canon.

any recommendations appreciated

[shooter]

start at the top of this page, there's a search function, type in 60hz hum, read through the posts.

tranny orientation, cabling, grounding, tubes, are just a few of the *usual suspects*

[Willabe]

Welcome! 

3. power tube cathode bias assembly (this is the one I currently suspect as needing to be separated given its proximity to signal... does this sound like something I should ground to another location?)

No. Leave it grounded with the PT B+ CT.

Here read this on grounding;      http://www.valvewizard.co.uk/Grounding.html

Can you post some high res gut shot pics?   

[centervolume]

Hi gents- thanks for those posts. The tranny orientation is not one I had heard before. I did perform the search as requested by the policies but only went about 30 results deep since the covered material is in keeping with my understanding. I realized however that I do need to verify the Hz - it may be 120. My meter appears to have a function for that, I'll investigate.

At this point,  I am wondering about the heater center tap from the power trans (green-yellow) that I have soldered directly to the chassis. Perhaps I should isolate that and use the "artificial center tap" approach (2 x 100 ohm resistors hanging from pilot light assembly)?


I'll take some shots of the circuit and post them for reference-

[Willabe]

At this point,  I am wondering about the heater center tap from the power trans (green-yellow) that I have soldered directly to the chassis. Perhaps I should isolate that and use the "artifcial center tap" approach (2 x 100 ohm resistors hanging from pilot light assembly)?

No, that shouldn't make any difference. 6 to 1, 1/2 a dozen to another, they both work equally well. 

[centervolume]

I did pull V1 and V2 (12ax7 type) and the hum persisted until I then pulled the 2x5881. Based on this info I am inclined to associate the hum with the power section; but wondering if this is a sound approach.

[DummyLoad]

schematic?


--pete

[centervolume]

this is all I have been working from, hopefully it is clear enough. laugh if you must, but it's my chicken scratch 

http://sclkssl.ssl.hwcdn.net/44/imgpages/1/4/4422021_775462.jpg?version=102

[Willabe]

No laughing, that's a very good hand drawn drawing.   

Nothing wrong with it, just a few little minor blips.

[centervolume]

taking the new pics, I just realized what I think is likely my error:

the main grounding point for the PT CT, cathode bias and power supply/filters is connected to the brass grounding plate - at the opposite end from the input jacks and pots, but they are all mounted to the same, 9 inch long plate

[centervolume]

here are some shots of this build, circuit-wise

here's the principal ground (PT CT, cathodes, filters) up on the brass plate (presumed error)

http://sclkssl.ssl.hwcdn.net/61/imgpages/1/4/4422021_775467.jpg?version=241

http://sclkssl.ssl.hwcdn.net/18/imgpages/1/4/4422021_775472.jpg?version=192

http://sclkssl.ssl.hwcdn.net/33/imgpages/1/4/4422021_775471.jpg?version=73

http://sclkssl.ssl.hwcdn.net/98/imgpages/1/4/4422021_775468.jpg?version=236

http://sclkssl.ssl.hwcdn.net/73/imgpages/1/4/4422021_775465.jpg?version=111

http://sclkssl.ssl.hwcdn.net/88/imgpages/1/4/4422021_775464.jpg?version=197

http://sclkssl.ssl.hwcdn.net/56/imgpages/1/4/4422021_775470.jpg?version=109

[sluckey]

I see two major issues with your schematic...

1... You have a 100K/10W resistor between the STBY switch and the OT primary. That resistor needs to be removed completely.

2... Your 50% switch is not wired correctly. In the position as shown V3 grid no longer has a grid return path. Also, 100K grid stoppers seems excessive. I'd use 1.5K for both those.

[shooter]

connected to the brass grounding plate

I would move this to your main chassis ground point

[centervolume]

shooter, that's my first order of business today... looking forward to that.

Sluckey- interesting you point that out. First, I abandoned the standby switch altogether in the actual build. There was a late addition to the drawing, namely additional 47uf/450VDC cap (that became the first in the power supply). I originally drew the schematic with just one 47/450 cap but another builder encouraged me to add the second with a 10 watt 100K in between the two 47uf caps. I had little space to draw this cap/resistor assembly in, as a result the cap is hardly visible. In light of this clarification, do you still advise dropping the 10 watt resistor?

Since this is a prototype design (working it out as I go along), is the final determination regarding 2 versus 1 ea. of the 47uf caps simply a matter of target B+ and whether or not the AC ripple has been eliminated? I'm getting 450+ on the plates of the 5881s which is a pretty decent pounding given the cathode bias arrangement, they are passing about 50ma at that voltage. I want the aggressive cathode bias sound but want to counterbalance with "somewhat" sensible parameters that won't require undue frequency of power tube replacement.

[sluckey]

If you are able to get 450V on the plate and 50mA current through the tube, then that 10W resistor cannot be 100K. With 450V the most current that could ever flow through a 100K resistor is 450/100,000=4.5mA. Surely that resistor is 100Ω, not 100,000Ω.  An extra filter cap plus a 100Ω resistor makes a lot of sense for improving filtering and reducing hum in a SE amp. Using a 100,000Ω makes no sense.

[centervolume]

my bad, you are correct: 100 ohms it is (file under "typo").

I moved the grounding point off the plate and the hum came down about 50% which is almost acceptable. I'm playing it and letting it burn in a little to see how things settle.

The proper switch wiring is my next order of business. Pin 5 grid on the 5881 is what I am using to switch from one power tube to 2 tubes. However where to install the 220K ground reference resistor is unclear. I have tried a couple locations and I am either able to get good performance in either single tube mode or both/parallel.

[DummyLoad]

please see attached.


--pete

[centervolume]

that's a thing of beauty pete. many sincere thanks for taking the time to do this. Do you mind if I ask what software allows you to generate such a pristine schematic?

by the look of the download count, I am not the only one who stands to benefit from your brainpower.

This should help stem my frustration... I am getting some odd spikes in the current of the output tubes (up to .2ma). As if the 25uf cathode bias cap has shorted.

[DummyLoad]

yw. it took just few minutes to draw. i use express.SCH. it's a canned application that comes with express.PCB software package. link below.

http://www.expresspcb.com/expresssch

-- pete

[centervolume]

want to double check a value on your detail that you drew:

the grid resistor on V3 pin 5 is labeled as 10K

the one of V4 pin 5 is labeled as 100K, correct?

Sluckey recommended I change those grid resistor values down closer to 1.5K each (in my schematic they are 100K, I was wondering if you put one of them as 10K in your drawing as a typo)

[DummyLoad]

they should both be 10k. sorry, i rush and make mistakes

--pete

[centervolume]

here is an updated schematic with voltages.

I still have the hum, but I think it is because of my power tube bias/current draw. With the 100 Ohm cathode resistor it was .2, I doubled the value to 200 ohms and it came down to .15; to get to where I would feel more comfortable based on other cathode bias amps I have worked on (about 75ma) I need to cut that approximately in half. My concern is putting too high of a value on the cathodes and the effect on the plate voltage going up beyond 465, 475+ volts 75ma of current will probably burn out 5881s pretty quickly.

http://sclkssl.ssl.hwcdn.net/05/imgpages/1/4/4422021_775541.jpg?version=229

[sluckey]

Some notes about your schematic...

1. You have a 47µF parallel with a 16µF for your first filter cap. That's a total of 63ΩF and seems a bit much for a tube rectifier.

2. You show 430v on the 47µF cap but on the same line you show 408v??? Is there actually a resistor between these two points?

3. You are dropping 408-280=128v across a 1500Ω/5W resistor. That resistor is really dissipating 128²/1500=10.9 watts! That resistor should be well done shortly.

[centervolume]

sluckey- yes, I am getting that impression as well... do they go well with A1 or "wooster" sauce?

clearly I am in need of a more effective, even elegant power supply for my design, the following responses are targeted toward that result

1. would 2 or 3 sixteens work better? is effective filtration a function (in part at least) of the number of filter caps?

2. I must have made a mistake on the drawing, there are a couple voltage drops on the front end: the output trans windings and the 1500 watt resistor (let me get in front of the chassis to verify my drawing)

[centervolume]

ok here are the actual power supply voltages (moving left to right from rectifier towards preamp)

B+, 432VDC at node of rectifier & 47uf@500vdc cap, feeds OT centertap goes to 1.5K 5-watt resistor
B+1, 411VDC on other side of 5 watt resistor, 16uf@475 cap, feeds power tube screens, goes to 75K 2-watt resistor
B+2, 189VDC on other side of 2-watt resistor, 16uf@475 cap, feeds V2b plate, goes to 22K 2-watt resistor
B+3, 143VDC on other side of 22K resistor, 8uf@450 cap, feeds plates of V1, V2a

the 5881s have 415 on the plates, 411 on the screens, 1.5ma of current

the heater CT is connected to the 200 Ohm resistor attached to power tubes cathodes as per bias setup 

[sluckey]

How much voltage is on pin 8 of the 5881s?

[centervolume]

pin 8, 5881:

30 voltsDC and .03VAC relative to ground

[sluckey]

30v across a 200Ω means 150mA flows thru the resistor. Assuming that current splits equally means each tube  is flowing 75mA. Real plate voltage is 415-30=385v. So, each tube is idling at 385x.075=28.9 watts. That's very hot for a 23 watt 5881. I would replace that 200Ω with a 470Ω and recheck everything.

[centervolume]

ok - will do. I may need to visit the electronics store in the AM, I only have 2 watt 470s now. I think 5 watts is a safe minimum based on the calculations you have shared.

Also, is the switch on my circuit a complicating factor here? I suppose I need to make sure that it is in the position that allows for both power tubes to be active.

[sluckey]

Switch is fine.

[DummyLoad]

ok - will do. I may need to visit the electronics store in the AM, I only have 2 watt 470s now. I think 5 watts is a safe minimum based on the calculations you have shared.

Also, is the switch on my circuit a complicating factor here? I suppose I need to make sure that it is in the position that allows for both power tubes to be active.

try this in the interim: see attached.


--pete

[centervolume]

Dummyload- I ended up doing that very thing, rather than waiting (got to use the time I get, which ain't much!)

I installed those at the same time as I put in your new switch wiring you graciously sent on your previous posts.

I'm going to have to go back over my installation of your switch circuit since I got the following results on the switch:
very good operation in the full operation (both 5881s) position; in the single 5881 position, I lost all my treble and
significantly more power than expected.

Sluckey - Regarding the voltages and current in the power supply with the improvised 6 watt 470 Ohm resistor, see below (some of these are counter-intuitive based on previous numbers, but the switch circuit overhaul did involve changing screen resistance to the current 2 x 220k setup and grid resistor values from 100K to 1.5K):

B+, 425VDC at node of rectifier & 47uf@500vdc cap, feeds OT centertap goes to 1.5K 5-watt resistor
B+1, 400VDC on other side of 5 watt resistor, 16uf@475 cap, feeds power tube screens, goes to 75K 2-watt resistor
B+2, 185VDC on other side of 2-watt resistor, 16uf@475 cap, feeds V2b plate, goes to 22K 2-watt resistor
B+3, 142VDC on other side of 22K resistor, 8uf@450 cap, feeds plates of V1, V2a

the 5881s have 411vdc on the plates, 27vdc on cathodes, 400vdc on the screens, 160ma of current is passing between OT b+ (red lead) and each power tube plate (is parallel operation this results in 80ma per tube?)

the heater CT is connected to the 470 Ohm resistor attached to power tubes cathodes as per bias setup. I am amending the schematic and will post shortly. Perhaps a layout drawing will also be useful to reveal problems in the way I am dressing the circuit on the board, pots, sockets etc.

OPERATION NOTES
the amp has good power and fidelity (i.e. tone controls operation) in the 2x5881 switch position, but the hum has not changed appreciably. While pulling tubes one by one from V1 up to V4, the hum does not disappear until I get to the 5881s, the math now shows me that the 5881s are now dissipating 30 watts (higher than before) so...

QUESTIONS
should I put an 820 Ohm cathode bias resistor?
since I have a 20 watt output transformer, should I replace the front end 47/500 cap with one or two 16/475 caps? perhaps the volts from the 47uf cap is contributing to my hum by creating operating 
            voltages that are too much for the overall design.
is there a rationale for locating volume control before or after tone stack in terms of signal flow from inputs to output?

[DummyLoad]

in SE amps a tube rectifier is worthless. since there is not much variation in current draw, sag is near impossible with SE. suggest you use SSR full wave, increase the filter size and add a RC network or LC network before the plate supply. otherwise with tube recto it's going to buuzzzzzzzzzzzzZZZZZzzzzz...... 


example here:


http://el34world.com/Forum/index.php?action=dlattach;topic=17818.0;attach=46940



--pete

[sluckey]

the 5881s have 411vdc on the plates, 27vdc on cathodes, 400vdc on the screens, 160ma of current is passing between OT b+ (red lead) and each power tube plate (is parallel operation this results in 80ma per tube?)

27v across a 470Ω resistor means there is 57.4mA. That current flows into the tubes. Hopefully it splits equally such that 28.7 flows into each tube. How are you measuring 160mA in each tube? 160mA would probably melt the plates.

[shooter]

fwiw, If you're buying parts, all my PSE builds, I give each PA tube cathode it's own R and C.
Makes it easier to *balance*, or notice if one tube is fritzin out

[centervolume]

that's the reading I got with the shunt method; used to taking reading on push-pull setups, less familiar with SE, so this is an exploration for me. 28-29 milliamps @ 385dvc per tube would be a lot more comfortable for these tubes.

here is a layout diagram. Excuse the nanofarad cap on the volume pot, that's not employed. I'm also still losing all my high end when I flip the switch to single tube operation. oh, and the rectifier b+ does not go to the cathode bias assembly, that's an error
in the illustration

http://sclkssl.ssl.hwcdn.net/21/imgpages/1/4/4422021_775571.jpg?version=240

For the buzz, my next steps are to move the OT from 45 degrees to 90 degrees relative to the PT
and to separate the ground of the 1st filter cap & PT HV center tap from the other caps and set it closer to where the PT is mounted, now its just off the brass plate perhaps still too close

[centervolume]

still troubleshooting, had some issues come up that I was able to deal with. Have yet to separate and move the 1st filter cap and PT center tap grounds from the preamp filters and ground closer to the PT. So the hum is still there.

The switch diagram, Dummyload, was much appreciated. Ever since I hooked it up I lose all my treble when I go to single 5881 mode. I'm thinking of hanging the 220K grid resistors to ground right off the sockets. I may have a bad .05 cap in there, have not replaced that yet (the one in there is a used one).

Getting good power and fidelity, power tubes no longer running past specified wattage.

Can someone check the hum frequency with a frequency analyzer if I post a sound clip of the hum? It seems that may be a tool for helping isolate the likely source of the hum if it is 60 or 120Hz

[sluckey]

I lose all my treble when I go to single 5881 mode.

That should not happen if the switch is wired correctly.

[tubeswell]

still troubleshooting, had some issues come up that I was able to deal with. Have yet to separate and move the 1st filter cap and PT center tap grounds from the preamp filters and ground closer to the PT. So the hum is still there.

The switch diagram, Dummyload, was much appreciated. Ever since I hooked it up I lose all my treble when I go to single 5881 mode. I'm thinking of hanging the 220K grid resistors to ground right off the sockets. I may have a bad .05 cap in there, have not replaced that yet (the one in there is a used one).

Getting good power and fidelity, power tubes no longer running past specified wattage.

Can someone check the hum frequency with a frequency analyzer if I post a sound clip of the hum? It seems that may be a tool for helping isolate the likely source of the hum if it is 60 or 120Hz

Some of it might be due to your ground return arrangement. Suggest you read the attached Merlin Blencowe article if you haven't already

[centervolume]

thanks so much tubeswell- I've had the thing on my desktop since you sent it and sent it to the printer downstairs... then I promptly left the office without checking the printer. NOw that's taking care of business 

I'll read straight away.

Have also been enjoying the wealth of resources on this site, man it is incredible all the work that went into building this

[centervolume]

I need a little help here with the 2 persistent issues, hum and the switch that activates the 2nd 5881 causing loss of high end and excessively choking the power.

Regarding the switch I have it wired exactly as Dummyload's drawing. I recall my initial setup was a much higher value with the grid stopper resistors, 100K v. current 1.5 K. In the parallel/2-5881 mode, the signal is a bit too bright, over the top gain; single 5881 muffled and choked signal with unpleasant distortion if pushed all the way with high end cranked. Perhaps some of the resistor value on the grid (1.5k, 220k) are relative to standard parallel operation and missing the mark in single tube mode?

Regarding the ground, I have read and implemented the recommendations regarding the grounding scheme in the power section as that is where it is coming from. The lack of results is making me wonder if I have sufficient filtering, if another cap is needed perhaps some attention to the reservoir.

any help appreciated as always

[pompeiisneaks]

If you can take some pictures, it will greatly help the guys understand what may be going on. 

As for hum, I for one have been battling it and learning it a lot.  Effectively, try to ensure you keep noise inducing things away from the preamp section as much as possible.  Another lesson I learned this month the hard way is keep your runs from the tubes to the board very short if at all possible.  (my fix was to replace all grid connections with shielded cables because I couldn't easily shorten the lead runs). 

Transformer placement, and/or PT and OT being in phase can cause hum.  Heater wires too close to input wires and/or not well twisted can def make it worse. 

I got a trick about measuring the hum from someone here on the forum, so if you post an audio sample, either they, or I will be able to do so yes, just make sure the sample is with the strongest hum you can find and don't have any other noise going on, so we can see if it is 60 or 120 cycle. 

~Phil

[centervolume]

I will post pics soon, I have linked a layout and schematic linked above.

I wanted to double check that the primary of the se 20 watt OT http://www.musicalpowersupplies.com/products/ is the best option for this application. In addition to the red lead on the primary there are blue 3k and brown 2.3k ohms choices . the one not used tends to be highly sensitive to noise and I am also curious about best practices for isolating it

[DummyLoad]

cut the unused plate lead a bit shorter than the other. cut the exposed conductor and cap it with heatshrink tubing. twist it up with the used lead and tape or tie wrap them together.


--pete