Fixed bias theory

[paggerman]

Well, now... It was mentioned that the original amp used 7591 tubes. In that case it's entirely possible that the bias wasn't using a doubler rectification circuit. The 7591. to me anyway, seems like a sort of cross beween a big bottle type power tube and an EL84. They don't require much bias voltage as compared to 6v6, 6L6 or EL34 type tubes. I don't know the specs for the tubes your lanning on using. But that will be an important thing to know when setting up the circuit for this supply. It's entirely possible that you won't be able to achieve the needed voltage without a doubler circuit.

And, to put the 56K resistor issue in the simplest light. The 56k resistor is one half of the voltage divider circuit that determines how much of the supplied voltage will be on the power tube grid/s. Nothing more. The value could be 100k or 22k if a corresponding change was made to the other resistor in the divider circuit.

Thanks Chuck, and everybody else who replied. I now have a deeper understanding of what's going on in electrical circuits. Still a long way to go...

[tubeswell]

My new build is using 6P3S-E power valves.

Good choice. I take you have seen the datasheet specs - 6P3S, 6P3S-E

Is there any reason why the original circuits (e.g. 5F6A) had no means of adjusting bias?

A plain 'ole resistor is cheaper and more reliable than a pot. And having come to that conclusion, Leo Fender probably thought that 'some sort of bias is better than no bias', therefore that he could get away with 'close enough is good enough'. (But I speculate)

Ah! That's what the 56k is for - a voltage divider. Got it. Thank you.

The voltage divider is the 56k and the 15k, which results in the divider's output voltage being about 4/5ths (78.87%) of its input voltage. So if you are using a fixed resistance divider, then whether you use a 56k and a 15k or not will depend on what the bias supply voltage is (as well as the B+ on your 6P3S-e )

[paggerman]

Thanks Tubeswell. So in the last week I have built a voltage doubler using a couple of 470uF 220v caps I had lying around (I think they came from a computer PSU) for the bias supply, which I designed on circuitlab.com. 4k7 and two 10uF in the pi filter and 100k pot to ground. Made all the jpreliminary tests before plugging in with a light bulb limiter. All good so far. Put the pre-amp 6n2P-EV valves in - so far so good. Voltage read a little high but I figure with no load from the power vlaves it's within expectations (about 20v high at the anodes). Popped in the 6P3S-E power valves, a speaker, the light bulb limiter and powered on.

Not a brilliant outcome. The noise was somewhat akin to the noise of a youth in a stolen Commodore's choice of in car entertainment. Farting, squealing, even the sound of an X-wing's laser cannon (how you can hear those in space is beyond me). All sorts.

I'm going through the process of elimination now. Power valves alone - dead silent. Add in PI - there's the problem. I've triple checked the construction of the PI circuit - all good. Next suspect is the cable I have from the treble wiper to the grid of the PI. I used a two core shielded cable. I soldered the two cores together (only grounded one end of the shield) and suspect there may be some capacitance thing going on between the two cores. Layout isn't optimal - my MO is to use recovered junk wherever possible and my PI is in the diagonally opposite corner to the treble pot.

The design is largely 5F6A - I have a doubler in the PS and a 6n2p-ev in the input, but otherwise very close.

Lets see what happens next.

Cheers

[kleuck]

Did you try first without any NFB ?

[tubeswell]

What kleuck said - disconnect the NFB and re-test.

Also, what bias voltage and tube current/plate voltage etc have you got on the 6P3S-e? (the 6P3Se is akin to a 5881 in performance, with similar operating voltages).

Re: the LTP, with just the LTP plugged in and not the other pre-amp tubes, you can sometimes get static-y crap coming through. But such noises with an LTP can also be associated with a dud presence pot or sup-otimal signal wire routing, or loose socket tube pins, or voltage board leakage coming from something like surplus dirty solder rosin between eyelets on your eyelet board, or over-soldering wrecking the contacts and resistive traces inside pots, or overheating CC resistors when soldering them in - Just some ideas for you to follow up with.

[paggerman]

What kleuck said - disconnect the NFB and re-test.

That would be easy to do. If it works, do I just leave it off?

Also, what bias voltage and tube current/plate voltage etc have you got on the 6P3S-e? (the 6P3Se is akin to a 5881 in performance, with similar operating voltages).

I set the bias to -48v prior to putting valves in, as per 5F6A. The B+ is in the region of 460v With no load. Haven't measured quiescent voltage or cathode current yet as I just wanted the noise to stop.

Re: the LTP, with just the LTP plugged in and not the other pre-amp tubes, you can sometimes get static-y crap coming through. But such noises with an LTP can also be associated with a dud presence pot have tried dialling fully CW and fully CCW with no change. or sup-otimal signal wire routing

I can assure you my signal wire routing is suboptimal ,

or loose socket tube pins

yeah the PI socket is a bit slack ,

or voltage board leakage coming from something like surplus dirty solder rosin between eyelets on your eyelet board

a good scrub with metho and an old toothbrush before starting build, going over it agin wouldnt hurt.

or over-soldering wrecking the contacts and resistive traces inside pots

with just the PI and power valves would a dodgy pot create its own signal?

or overheating CC resistors when soldering them in

no Cc resistors carbon film and metal film and ww only- Just some ideas for you to follow up with.

thank you so much. We've gone way off topic, and didn't ask for help, but you have given me some great pointers.

Thanks as always

[paggerman]

What kleuck said - disconnect the NFB and re-test.

That would be easy to do. If it works, do I just leave it off?

Also, what bias voltage and tube current/plate voltage etc have you got on the 6P3S-e? (the 6P3Se is akin to a 5881 in performance, with similar operating voltages).

I set the bias to -48v prior to putting valves in, as per 5F6A. The B+ is in the region of 460v With no load. Haven't measured quiescent voltage or cathode current yet as I just wanted the noise to stop.

Re: the LTP, with just the LTP plugged in and not the other pre-amp tubes, you can sometimes get static-y crap coming through. But such noises with an LTP can also be associated with a dud presence pot have tried dialling fully CW and fully CCW with no change. or sup-otimal signal wire routing

I can assure you my signal wire routing is suboptimal

or loose socket tube pins

yeah the PI socket is a bit slack ,

or voltage board leakage coming from something like surplus dirty solder rosin between eyelets on your eyelet board

a good scrub with metho and an old toothbrush before starting build, going over it agin wouldnt hurt.

or over-soldering wrecking the contacts and resistive traces inside pots

with just the PI and power valves would a dodgy pot create its own signal?

or overheating CC resistors when soldering them in

no Cc resistors carbon film and metal film and ww only- Just some ideas for you to follow up with.

thank you so much. We've gone way off topic, and didn't ask for help, but you have given me some great pointers.


I haven't figured out how to break up your last post so ocean respond to each point you have made in line, but if you read between your lines you'll see my comments.

[tubeswell]

The presence pot is part of the LTP tail. If there is voltage leakage around the pot contacts, it can wreak havoc with the LTP (I know because this has happened to be before - as has sub-optimal signal wire layout, and loose socket pins)

[paggerman]

Did you try first without any NFB ?

I had very limited time over the weekend to work on this, but did have a chance to lift the NFB. There's only one type of noise now, so I'd say that was a partial success. The only noise left (that I can hear for now) is a loud buzz, probably 50Hz, unaffected by twiddling any knobs. I will need to revisit my grounding scheme using the advice of Aiken, Blencowe et al.

But what do I do with NFB after that? Just remove it?

When turning the presence pot there is a 'scraping' noise - not having used such a control before, is this normal?

Thank you.

[kleuck]

If the nfb is reverse-connected, the amp squeals, if the nfb is too "strong", the amp oscillates, so, if there is an issue, first thing to do is to remove the nfb. The presence cannot work normally without nfb, don't worry. When others issues are cured, you will try to reverse the connection to the OT, will probably make the nfb fully functional.

[paggerman]

Just thought I'd give an update on my progress with this build. I decided that layout was crap,my wiring was crap and the the valve sockets (from the original donor chassis) were crap. So I took the whole thing apart and drilled new preamp valve socket holes and put in new Belton power valve sockets. I built a voltage doubler with a pot for the fixed bias. At some stage of the rebuild I realised I had ALL my pots reverse wired so I have sorted that out. I am getting much better results now. I have had to disconnect the NFB as I was getting a regular popping sound. I'll try reversing the polarity as per kleuck's suggestion. I'll update soon. Thanks to all.

[paggerman]

So I swapped around the output connections and reconnected the NFB, set the bias for -48v and plugged in. It's a result. I have three 6n2p-ev in the pre amp and PI and a pair of used Sovtek 6L6-GC (that look exactly like the 6P3S-E valves I have, right down to where there are pencil signatures on the anodes). Into a test speaker (taken from an old Kawai organ, no cabinet) at low-ish volume it sounds really sweet. I have two 4x12 loaded with Rola 12PEG speakers to try it out with. I have no reference to tell you what it sounds like compared to a 'proper' 5F6A but I'm really pleased with it. Thanks to everyone for their help in what has been a long drawn out build.

[Chuck H]

Looking forward to reading about the acid test... Cranking it up!