Rhk for a JTM45 "Replica"

[Baronkarza]

Hi everyone.

I have to pass the time of the lockdown somehow,so i decided to build , among many other things i have to do, a "replica"( clone-someone) of the jtm45 with 6n2p-ev at place of 12ax7 and 6p3s-e at place of EL34/KT66/5881 etc etc.

The tubes sharing the same heater voltage ( transformer 3.15-CT-3.15)

I just made a few mods:

1) Tube Rectifier diode mod
2) Put 1R at cathode of each power tube for check bias
3) Diode arc protection for cathode follower
4) Grid stopper input jack directly soldered at tubes with shield wires
5) And of course, I rewired the filament of preamp tubes ( 9 pin in 6n2p-ev is a internal shield to be grounded)

I must check all voltage without tubes and with tubes but I have to make some adjustments (after checking the voltage with only the tube rectifier, to have the voltage values to start from)

I use 6n2p-ev at place of 12ax7, so the voltage seen at cathode of Cathode-Follower (theoretically on schematic 180 V ) is too high for Vhk of the 6n2p-ev.
So, i have to elevate the heater.
The purpose is:

First : lower the cathode resistor of CF from 100k to maybe 56k-47k.The voltage would change from 180V to maybe 165V-155 V.( i think any particular changes in sound)

Second :elevate the CT of heater voltage in range of 65-75 volt, to stay below the value of Vhk max ( 100V) for a 6n2p-ev .
The partion voltage divider formed with 220k (R1)( bleed R) and a 56k (R2) ( with a paralleled 33uF 100v at 350 volt of B+ ) will do the job.

The 6p3s-e ( from the poor and conservative datasheet that we found on web) voltage max Vhk ( positive) is 90 V. So with 65-75 volt I should stay in the ballpark.


I know datasheets are pretty conservative at times.( and 6p3s-e is an example).
So.
In the data sheet of EL34 (pentode), and only in the data sheet of EL34, in case of fixed bias, not only the Vhk must say at 100v max but the Rhk ( the R2 of partition voltage divider) must stay at 20k max.
The 6p3s-e is a beam tetrode.

And now the question.

Does the rule of 20k Rhk apply to the case of 6p3s-e ?
Or 56k is safe and EL34 is a particular case ( El34 is a little tube compared to "large" 6p3s-e) ?

I have also read other datasheets, but no reference is made to the max Rhk resistance.



Thank you in advance
And stay safe

Emiliano

[Helmholtz]

First : lowering the cathode resistor of CF from 100k to maybe 56k-47k.The voltage would change from 180V to maybe 165V-155 V.

No good idea. The cathode voltage of the DC coupled CF is essentially identical (maybe higher by 1V) to the plate voltage of the preceding triode. It's a DC follower.
Lowering the cathode resistor will force the CF into grid conduction but hardly lower cathode voltage.
You might try a higher value plate resistor but that may change sound.

In the data sheet of EL34 (pentode), and only in the data sheet of EL34, in case of fixed bias, not only the Vhk must say at 100v max but the Rhk ( the R2 of partition voltage divider) must stay at 20k max.
The 6p3s-e is a beam tetrode.

If it's not mentioned in the tube data, it's probably not critical.
If you want to play safe, lower R2 to 20k with R1=100k. Needs 1W resistors.

Please post your schematic and the tube datasheets.

[trobbins]

That Rhk parameter pops up in some datasheets. The only description I have come across for it is in this 1962 article:
https://www.dalmura.com.au/static/he...insulation.pdf

So it appears to be the maximum safe resistance in the circuit for situations like if the cathode is disconnected, but would seem to also need to cover cathode follower application. As Helmholtz says, if it is not in a specific datasheet for the part you are using then it may not be a limitation for operation as long as the other limits are met.

[Baronkarza]

Hi,
First of all thank you for the answers and sorry for my very bad english.
I do rewrite the post in a better form

No good idea. The cathode voltage of the DC coupled CF is essentially identical (maybe higher by 1V) to the plate voltage of the preceding triode. It's a DC follower.
Lowering the cathode resistor will force the CF into grid conduction but hardly lower cathode voltage.
You might try a higher value plate resistor but that may change sound.

What if I tried to lower the cathode resistance from 1k to 680 ohm of the first triode in order to slightly change the bias point?
The cathode voltage of the second triode and the anode voltage of the first one they would be the same ( maybe 165 volt).
The cathode voltage of the first one ( the gain stage) would drop from 1.2 volts to 0.9 volts.

It's right ? Or am I wrong again?

If it's not mentioned in the tube data, it's probably not critical.
If you want to play safe, lower R2 to 20k with R1=100k. Needs 1W resistors.

Please post your schematic and the tube datasheets.

Not critical. I tought too.

This the schematic
https://www.tube-town.net/cms/userfi...tm45-schem.pdf

The 6P3S.pdf is the same of Reflector_6n3c-E - Datasheet, but in english .

Thank you again

Emiliano

[Baronkarza]

That Rhk parameter pops up in some datasheets. The only description I have come across for it is in this 1962 article:
https://www.dalmura.com.au/static/he...insulation.pdf

So it appears to be the maximum safe resistance in the circuit for situations like if the cathode is disconnected, but would seem to also need to cover cathode follower application. As Helmholtz says, if it is not in a specific datasheet for the part you are using then it may not be a limitation for operation as long as the other limits are met.

Thank you for the very interesting article.
Have a nice day
Emiliano

[Helmholtz]

Hi,

What if I tried to lower the cathode resistance from 1k to 680 ohm of the first triode in order to slightly change the bias point?
The cathode voltage of the second triode and the anode voltage of the first one they would be the same ( maybe 165 volt).
The cathode voltage of the first one ( the gain stage) would drop from 1.2 volts to 0.9 volts.

I see, so you increased the plate current of the gain stage to lower plate voltage.

[J M Fahey]

Thinking laterally: you may replace the cathode follower with a Mosfet, lots of examples doing that, I think even some modern Marshall does that.
No filament

[Helmholtz]

I would be more concerned about exceeding the 6P3S screen (300V) and plate (375V) limits in a JTM5 than the Vhk limit in the DCCF.

[Baronkarza]

Hi everyone and Happy Easter.

The power tubes are 6P3S-E.

I made the heater elevation and changed the cathode resistor of DCCF to 680 ohm. Now i have 166 volt on cathode follower and 70 volt on heater elevation with a voltage divider with 220k-56k( 10uf paralleled)

I have 435 volt on B+ and 433 on Anode and 431 on screen.
I adjusted the bias pot to -49 volt with 31 mA bias for one tube and 29 mA for other tube.


I put a 1kHz 100mV p-p sinusoidal signal in the Normal Channel. At the output i have a clean sinusoidal signal at 38 V p-p on a 8 ohm load.
So i should have about 23 Watt clean before distortion. And about 28 Watt maximum with some distortion.

I plan to adjust the bias to -43 volt, measure again the Bias Currents and Plate Voltages.
After that , if all is ok, measure again the heater elevation and cathode voltage of DCCF.
Maybe change the voltage divider to 220k-62k ( 10 uF paralleled)
Maybe change the OT primary from 8k8 to 6k6.
No sign of Red Plating on 6P3S-E


Thanks again to everyone.

Emiliano Ranfagni

[trobbins]

Emiliano, be mindful that a fuse in the CT can cause damage to the bias circuitry if the fuse blows. In general, a better approach is to fuse the leads to the 5AR4 plates (but after the bias feed tap) and keep the CT un-fused. You should then re-assess the fuse value - the link below may help on that.
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

[Baronkarza]

Hi,
I understood what you say.
In case the secondary fuse blows, at R104, i should have double the off-load voltage in AC ( 1000 Vac) and damage the bias circuitry.
Not he tubes. I think.

Not a problem to put after R104 and before R108 a fuse holder
And other one fuse holder before R109.
The half of the value of CT fuse, does the job ( 250 mA for anode)
I have two of these ( see photo)

Old School. One fuse = save money. Got It.

Thanks
Emiliano Ranfagni