Building parafeed EL84 DC 2A3 amp

[euro21]

Somehow my 801a's that I have are sensitive like hell...I had to remove power transformer from the chassis. Kinda soured me to the 801a. Perhaps I should try it again now that I have a bit more experience.

Each old DH tube sensitive to the vibrations, some more, some less ... but all of them requires vibration damping.


As you can see in my 801 preamp, I always use separated box for PSU/s/ and preamp.
The tube sockets is in the subplate, decoupled from the top with rubber isolators/vibration dampers (they are dimly visible on the black top plate).

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[Bas Horneman]

Had three seperate boxes. One for filament transformers and power supply boards. One for the HT.

Still it was a hot mess. I suppose I should try it again one day.

 

[troposnet]

To try out an SE design, I have taken a push-pull transformer and placed B+ voltage at the center-tap of its primary. 1/2 of the winding went to the tube's plate, and the other half went to a constant current source set to the same current as the tube draws. That arrangement canceled out the magnetic field in the transformer's core, and since the constant current source has a load impedance in the megohms, it did not load down the tube. This eliminated the capacitor and the choke in the original design. That arrangement also allowed for the use of a smaller, non-gapped transformer.

egellings, would be interesting if you could post up yr circuit diagram which was used for this arrangement. im currently working on trying to use dissimilar devices (ie. tube & mosfet) with a push pull output transfo. thank you

 

[Bas Horneman]

Added the Hazen mod (0,1uF cap from pin 1 or 6 to cathode.) Does not affect EL84. But is said to make an SV83 sound better.
I have now tried EL84, 6P43P-e and SV83 in that position. (But not done any thorough comparison)

 

[dubadub]

SV83, amplifier tube for video frequencies, has g3 connected to pins 1&6. EL84 and most audio tubes have g3 internally connected to Cathode. If there was a benefit to decoupling g3, wouldn't more audio tubes have the option?

 

[6A3sUMMER]

The SV83 Maximum Screen Voltage is 200V.
The EL84 Maximum Screen Voltage is 300V.

Can anybody tell us what the Maximum Suppressor Grid Voltage is for the SV83?
Yes, I thought so, no answer.

Perhaps the Hazen mod (no DC connection to the Suppressor Grid), when applied to a SV83 works OK, perhaps Not OK.
But the purpose of Power Pentode's Suppressor Grids has Always been to Capture Secondary Electrons that come off the Plate.
The fact that some designers choose to use the Suppressor Grid for another purpose is up to them (Their Mileage May Vary).

A floating Suppressor Grid can not continuously collect Secondary Electrons (there will be secondary electrons), then pass them off to a 0.1uF capacitor.
When will the voltage of the Suppressor Grid and the capacitor stop changing (perhaps not stop changing)?
What is the range of voltage on the Suppressor Grid over time; it sounds like a case of Uncertainty to me.
It is like causing the suppressor to become a Modulator Grid, with no known results.

I was very reluctant to Suppress my opinion of the Hazen Mod, it all seems too Hazy to me (puns intended).

Marketing Died. Marketing came back to life. Long Live Marketing!

 

[Bas Horneman]

Yeah. Bottom line is I wanted to try it for myself. My Zen clone with hazen mod sounds lovely. (did not try it without though)

The SV83 sounds too forward in this amp. 6p43p-e very little gain and darker sound.

EL84 best of the lot. So EL84 it will be for me.

 

[dubadub]

I got some 6P15P-EV from eBay, put em in my little UL-SE EL84 amp with a wire connecting Pins 6&3. To me they sound better than JJ EL84 and 6P14P-K. They aren't quite SV83 tho, the datasheet gives 330v for Ua and G2.

 

[ggetzoff]

Yeah. Bottom line is I wanted to try it for myself. My Zen clone with hazen mod sounds lovely. (did not try it without though)

The SV83 sounds too forward in this amp. 6p43p-e very little gain and darker sound.

EL84 best of the lot. So EL84 it will be for me.

How are you liking this amp after almost a year? Great work!

 

[Bas Horneman]

Thanks. Yes still liking this amp. I did change the parafeed cap. As the continental cap sounded a bit closed in. Switched to a smaller value Jantzen Superior. ( I liked a Russian pio more. But it would not fit.)

 

[nerdorama]

I think the SV83 may be a relative of the 6p15p. There is an interesting post about using some positive voltage on the 6p15p suppressor of around 25-30VDC. Apparently it squares up the left side of the curves and also reduces screen current at the low voltage side of the curves so it works more like and EL84. The discussion by @Bluesystems begins at post 10. The Hazen mod might do something a bit similar to this by letting the suppressor voltage float upward but this post makes more sense to me and is documented as to the results.

B

Post in thread 'Using 6P15P in triode at 300V - suggestions?'

2024-02-16 7:16 pm

What I found with the 6P15P at higher screen voltages is when driven hard the screen current can become much higher when the anode voltage drops below 100V than for example a 6BQ5. The result then with a higher screen voltage is very high screen dissipation when the amplifier is driven deep into clipping or loaded with too high a output impedance. Outside of high anode current at lower anode voltages it behaves quite well in 6BQ5 type circuits in my experience. This could explain why some find the 6P15P works just fine while others find it dies from screen failure. The tendency to draw...

• Bluesystems

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[Bas Horneman]

I think the SV83 may be a relative of the 6p15p.

AFAIK it is the same tube different name.

 

[mctavish]

Beautiful amp Bas!!!😎👍🏻

 

[Windcrest77]

Nice project! How has the first choke been holding up since you've been using the amp? Considering it is "nearly" being a choke input supply with such a small input capacitor of .47uf and, and being that it is a solid state rectifier. Can I ask where you sourced the 10H chokes?

 

[Bas Horneman]

The first cap is to "help" the first choke. It's life was much harder before the cap. I got the chokes from a seller here on diyAudio. They were made in Croatia. By Mladen Krklec who used to have a site Sound-Carrier but the site has been down for a few years now.

So I guess it's still holding up.

 

[ggetzoff]

Bas, How does this compare to your BabyHuey (if you still have it)?

 

[Bas Horneman]

I know I've raved about the bh. But living with it... pp is not the thing for me or perhaps the bh. I tried twice. Incomperable really.

 

[ggetzoff]

I've got lots of EL-84s and a pair of Sovtek 2a3s........