Adapting an 2xEL84 amp to use 2xPL83

[dougietamson]

In the 'lab' today with mad mad prof lab coat and breast pocket stuffed with pencils and screwdrivers...

A while back I bought a B9G base valve storage box, it also came with a range of 1950s used valves (nothing for the audiophools to get worked-up about) mostly series heater Pxxnn types from TVs, it was only £2 and I did just buy it for the storage and some spare PCL83 for my Pye Black Box.

This is where I got a couple of used Mullard PL83, Blackburn 1958.

A quick look at the datasheet and there isn't much work to do to try them in my push-pull EL84 20W guitar amp (fixed bias with a trim pot to adjust the bias voltage) goal is to make it easy to swap out at the flick of a DPDT switch and a bit of heater adjustment.
Lower max power output but still plenty.
I tied pins 3 - 6 with a jumper and added the switch to tie pins 1 and 9 when EL83 or PL83 are used.

While I had the chassis on the bench I added some RCA sockets at the back for bias prob points, I'll measure the voltage drop from each anode to CT on the OT (doesn't include any screen current but should get me in the right area).

That's easy bit done, filaments next...

The amp is making it a bit easier for me, the pre-amp and power amp sections have dedicated filament supplies
Pre-amp section of 5 x 12ax7 inc phase inverter have 12.6v dc. The output valves have 6.3v ac, the 2 EL84s should draw 1.5A so we have at least 6.3v at 1.5A to play with.
To use PL83s the plan is to convert the 6.3v ac to dc and then use a DC-DC XL6009 based booster 3-30v input to 5-35v output. I bought 5 of these on Amazon for £8.99.
If I had a pair of EL83 valves on 6.3v filaments, I'd just need to adjust the bias.

Today I tried it with the 2 x PL83.
I switched the amp to standby with output valves removed and set the trimmer on the DC-DC boost module to 15v (no load).
I setup 2 DMM to read the voltage between pins 4 and 5 and also the current draw of the 2 valves parallel.
I set the final voltage based on the current draw, 14.5v gave 600mA.

Any thoughts on set by voltage or current, I'm thinking about possible issue if inadvertently running them cathode starved.

If all goes well, down the road I might splash out £10 on a pair of NOS Mullard PL83 or even break the bank at £14 for 2 x EL83.

JJ EL84 are £20 each so more money in my sporran

Doug

[G6Tanuki]

I wouldn't fret too much - TV valves were for series-heater use and so it was quite normal for the voltages across the valves to vary by something like 5% either way and theu seemed to survive OK.

[A house my rellies had was powered with 210V DC from the local pit; the voltage fluctuated quite a bit depending on what was going on, they got used to fiddling with the line/frame-hold controls quite a few times during an evening's viewing].

[dougietamson]

Thanks,
you can imagine Dad telling Mum to wait till after the football before putting the spin dryer on if she's also got his dinner in the oven or suggest she uses the mangle. The mind goes back to a situation similar to a scene in 'Till Death Us Do Part'

Also, just to check, the PL83 is plentiful and no issue with using other than in vintage TVs? I'd imagine many other parts being a lot more of an issue.

The PL83 is labeled Ferranti but has the Mullard Blackburn date code.

Doug

[murphyv310]

Hi
I'm a bit confused. The amp had EL84 but you fitted PL83 are you sure about that? The PL84 is pin compatible.
Wired in parallel the heater current should be 600ma.
The PL83 is a video output valve and the PL84 either frame or audio output.

[dougietamson]

Yes, the PL84 is pin compatible, but I don't have any of these but I do have some PL83 valves, video output valves, but well built.
The goal was to find a use for the PL83's that had been gathering dust and I got for free.
I'd read that the amp tech that worked for Rory Gallagher, a well respected 60/70 guitarist, fitted EL83's with a few modifications, so I had the idea to use the series heater version.
The result sounds very good to me, I've not played them at high volume yet.

Doug

[whiskas]

Hi Doug,
Apart from the obvious change to the heater supply, what were the other finalised changes (cathode bias, etc) that you made for the PL83 to work successfully?
I have a few PL83's here and never thought to re-purpose them, but your post has inspired my interest!

[dougietamson]

The amp I modified uses fixed bias so no cathode bypass resistor, the anode voltage is on the high side at 380vdc.
I'll measure the -ve bias voltage and anode load to get some ball park figures.
If you go for a standard EL84 pp cathode bias/self bias circuit, I think the resistor value would be towards the 'cold' end, always better to start cold and bias towards hot, measure the voltage drop across the cathode resistor and calculate the current draw.
The PL83 datasheet shows Ik max of 70mA and Pa max 9W vs 65mA for the EL84 and Pa max 12W. The higher cathode current I assume is due to the harder service duty in a TV set.

Sounds like a good project.

Doug

[dougietamson]

Originally Posted by whiskas

Hi Doug,
Apart from the obvious change to the heater supply, what were the other finalised changes (cathode bias, etc) that you made for the PL83 to work successfully?
I have a few PL83's here and never thought to re-purpose them, but your post has inspired my interest!

Here are the readings I made today:
HT supply:
CT of OT is at 377v
anode at V6 is 375v
screen at V6 is 360v
anode at V7 is 376v
screen at V7 is 359v

The -ve bias at V6 grid is -8.03v and -7.94 at V7 giving:

V6 anode to CT vdrop = 1.44v - DCR of primary A1-CT is 54.7Ω = 26mA
V6 screen resistor is 468Ω vdrop = 1.69v = 3.6mA

V7 anode to CT vdrop = 1.03v - DCR of primary A2-CT is 43.1Ω = 23.9mA
V7 screen resistor is 467Ω vdrop = 1.42v = 3mA

So combined current is 29.6 for V6 and 26.9 for V6. If you measure the cathode resistor vdrop/Ω you get current for a self bias amp, it will include the anode and screen current combined. For a fixed bias amp you can insert a 1Ω resistor between the cathode and measure the vdrop and no mental arithmetic required

Attached schematic.
Doug

[Richardgr]

I converted a Tubelab SPP from EL84 to PL84 and t turned out to be more complicated than I had bargained for.

The PL83 looks to be closer to a PL84 than an EL84, so I wonder if you will hit the same snags. The TV tubes all seem to have 250VDC max ratings for anode and screen. The PL84 was just 200VDC, and 170VDC in some tube data notes. (Which is another issue, TV tubes seem to have a lot more variation between suppliers).

I used the correct OPT for PL84 (3K a-a), and lowered the B+ to around 250, and it sounded great when first converted, but I ended up with red-plating after a few minutes. I just could not get the PL84 to work with higher B+ with UL. I would like to try with a regulated screen grid one day, but I really wanted to use my bespoke OPTs I had had made by Toroidy.
Initially I tried to drop the B+ so that the output tubes were happy, but then I lost the headroom I needed for the driver and splitter (ECC81).

In the end I had to adapt the power supply so that there were two supplies:
220VDC B1+ for the output tubes
280VDC for the driver tube/splitter.

Then it performed very well every day for some months.

[dougietamson]

Originally Posted by Richardgr

I converted a Tubelab SPP from EL84 to PL84 and t turned out to be more complicated than I had bargained for.

The PL83 looks to be closer to a PL84 than an EL84, so I wonder if you will hit the same snags. The TV tubes all seem to have 250VDC max ratings for anode and screen. The PL84 was just 200VDC, and 170VDC in some tube data notes. (Which is another issue, TV tubes seem to have a lot more variation between suppliers).

I used the correct OPT for PL84 (3K a-a), and lowered the B+ to around 250, and it sounded great when first converted, but I ended up with red-plating after a few minutes. I just could not get the PL84 to work with higher B+ with UL. I would like to try with a regulated screen grid one day, but I really wanted to use my bespoke OPTs I had had made by Toroidy.
Initially I tried to drop the B+ so that the output tubes were happy, but then I lost the headroom I needed for the driver and splitter (ECC81).

In the end I had to adapt the power supply so that there were two supplies:
220VDC B1+ for the output tubes
280VDC for the driver tube/splitter.

Then it performed very well every day for some months.

The figures I gave were pushing it to the 9W limit, no red plating and keep the screens below the 2W max.

A change I.m thinking of making is to add a proper heatsink to the DC-DC boost module as it will be inside a closed chassis.

I'm also testing another pair of Mullard PL83 valves, I knew I had 4 of them but couldn't find the other 2, one is from 1956.

Doug

[Sideband]

TV valves 'P' are generally lower rating than the 'E' versions. The main HT supply in TV's was generally lower than that in an amplifier or radio so the valves had to work at lower HT supplies. Another reason was that a lot of mains supplies in the UK were 200V and when the demand was high, the supply could drop down to 180V. The valves had to be designed to cope, within reason, of even lower HT when that happened.

If you look at valve data books, you'll find that most 'P' valves have lower anode and screen grid ratings. So a PL84 is not an EL84 with a different heater!

[Richardgr]

Originally Posted by dougietamson

...
The figures I gave were pushing it to the 9W limit, no red plating and keep the screens below the 2W max.

...

Doug

Hi Doug,

Is this a planned change or does it work OK now? The PL84 mod I did was also within spec, just that having the screen above the max rating caused the red plating. The higher value of screen grid resistor is some insurance.

[dougietamson]

Originally Posted by Richardgr

Hi Doug,

Is this a planned change or does it work OK now? The PL84 mod I did was also within spec, just that having the screen above the max rating caused the red plating. The higher value of screen grid resistor is some insurance.

Yes, might be a good idea to up the screen resistor, though the power dissipation is inside the max range. The screen voltage is well over spec.

Does anyone know what the Vg2o from the datasheet refers to?

Doug

[ms660]

Originally Posted by dougietamson

Does anyone know what the Vg2o from the datasheet refers to?

Vg2o means Vg2 with no current flowing, eg: valve at cut off or still cold (cathode current approx. zero)

Lawrence.

[Richardgr]

As a matter of interest, people always say the difference between the E- and P-series tubes is just the heater, but I do not believe that. I think the E-series tubes were tougher, or made to a slightly higher spec.

Here is the EL83 spec sheet as an example showing 300V limits, not 250V ...

It was the same when I was using PL84's instead of EL86's.

[kalee20]

Originally Posted by Richardgr

As a matter of interest, people always say the difference between the E- and P-series tubes is just the heater, but I do not believe that.

You're right not to believe it, it's the exception rather than the rule that they are the same!

PCL83 and ECL83 for instance have very similar pentode sections, but quite different triodes. One counter-example!

For here, the PL83 and EL84 have different maximum anode dissipations, as already noted - and they are not pin-compatible. But, mutual conductance is similar.

If the original EL84's were running at 9W or less (Uncommon actually, people thrashed their EL84's) then with a rewire, a bias adjustment to achieve the same current, and appropriate heater supply, you can virtually substitute the PL83 for the EL84. If not, then you'll have to accept a lower output power, you'll have to adjust bias (or cathode resistor) to bring dissipation from 12W to 9W... and then really you should raise the load on the valves by the same ratio.

[dougietamson]

Originally Posted by kalee20

You're right not to believe it, it's the exception rather than the rule that they are the same!

PCL83 and ECL83 for instance have very similar pentode sections, but quite different triodes. One counter-example!

For here, the PL83 and EL84 have different maximum anode dissipations, as already noted - and they are not pin-compatible. But, mutual conductance is similar.

If the original EL84's were running at 9W or less (Uncommon actually, people thrashed their EL84's) then with a rewire, a bias adjustment to achieve the same current, and appropriate heater supply, you can virtually substitute the PL83 for the EL84. If not, then you'll have to accept a lower output power, you'll have to adjust bias (or cathode resistor) to bring dissipation from 12W to 9W... and then really you should raise the load on the valves by the same ratio.

Here's a doc for converting EL84 based amps with a TV valve, the 6P15P aka SV83

PDF saved from here:
https://web.archive.org/web/20160415...lletin_29.html

Doug