PP3521 idht

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Paul Barker
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#16 Re: PP3521 idht

Post by Paul Barker »

Iv’e found 3 mesh 27’s
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#17 Re: PP3521 idht

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So Ive been calculating (sadly Ive lost the A “(A)ctual gain” spreadsheet I made to speedily come up with A which is less than mu)

So in RCL coupling that I plan for two stages of 27:

1/ the load is anode load resistor parallel with grid choke. The worst case scenario load would be the max permitted grid resistor 150,000 ohm. I know the grid choke is greater equivalent than that. Parallel with load resistor 47,000 = 36,000 ohm

2/ Im not bypassing the cathode resistor (360 ohm) so , quick digression to compund curves internal resistance is 11,000 ohms I think effect of unbypased cathode resistor is mu+1 x rk added to internal resistance mu from compound curves 8.5 (9.5*360= 3420+11000 = 14420) source resistance.

”A” becomes 6.07 per 27 stage squared for two identical stages = 36.84

Working backwards from pp3521 max power requires 25v peak 17.7 v rms / 36.84 = 0.48v rms, as this is more likely >/= 1vrms we have enough headroom and can go without cathode bypass caps.

I decided the idea of IT’s would require expense by the way, and involved compromise to keep the expense not too great. Any expense is too great in the end, the parts for rcl I have already.

There are two coupling caps. Im going to try paper in oil first.

I also have 4 nos 37’s, but have a fourth mesh 27 on order, so 37’s are only in reserve.

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#18 Re: PP3521 idht

Post by ACPEN »

Hi - I too have tried the PP3521 in SE using drivers recommended to drive PX4's but was never happy with the results - these were the H42 and AC/S2PEN types.
Since seeing this thread I decided to try again using MH4's as drivers- good,
but slightly low gain, I then tried MH4's in mu follower configuration - the outcome being higher gain and lower output resistance - this did the job and well worth a try if you haven't settled on other means yet, but don't overlook elevating the heaters.
Rgds
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#19 Re: PP3521 idht

Post by Paul Barker »

Thanks for the input. Srpp totem pole mufollowers ccs’s pentode ccss’s I havent used for over 23 years, not my bag but if I fail I’ll know why. I’ll keep it simple first. If it needs lower impedance drive it can get another way of reducing that. Trying to stick to idht’s and would rather use 1920’s 1930’s idht’s. But i could easily cheat with 6em7 power section driving the output valve. I’ll not do that unless necesary.

So with sufficiently low impedance drive did you like the pp3521?
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#20 Re: PP3521 idht

Post by ACPEN »

Yes, I like the PP3521 and have done for some time, also the PEN3520 which although a pentode makes a reasonable sub and can be a direct plug in.
I too have used the USA types, UY-227 and UY- 224 (tetrode) but came back to the British types of the same period, MH4 being also from the 30's. Be interesting to hear your thoughts should you stray into SRPP or Mu follower territory. I should say that I also tried the pentode mu follower version but decided against - I find the triode Mu follower 'sound' very similar to the common cathode RC configuration.
Good hunting!
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#21 Re: PP3521 idht

Post by Paul Barker »

Thanks.
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#22 Re: PP3521 idht

Post by ACPEN »

Just to say that have broken with tradition (my own) and have tried a 'modern' approach as a driver - an E810E ! in pentode mode - 10k load, 150v S.G, 100R bias. 200v ht, runs at approx 14mA - sounds Good, certainly worth a try if anyone is still looking for driver options for the PP3521 triode. To be honest after listening to 300b amp for some time, very little difference.
Caveat, my ears are 81 yrs old!
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#23 Re: PP3521 idht

Post by ACPEN »

Ooops!
E810F or E180F pin compatible if you leave pin 6 disconnected.
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#24 Re: PP3521 idht

Post by ACPEN »

Hi Paul
See you're back onto this ( disguised as GM70)
Just to say I persevered with it and via trying AC/SP3's, settled on the later SP61's as a good option, being MAZDA and good spec.
270 R bias, 50k load. 105v screen.
Worth a try...and very cheap - £4.00 @ Langrex!
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#25 Re: PP3521 idht

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I was checking my maths on the d3a version and discovered I’d made an error from the beginning, it wouldn’d have suited the Hammond interstage, too much internal resistance. Tried to make the 5687 work, but too low mu. So checked the suitability of ECC88/E88CC Which just makes the minimum requirements of no more than 15 mA dc current through the Hammond 126C interstage, yet enough input sensitivity to drive the grids of PP3521 pair to full power output, which requires 1.3v rms from the source. The sound of other valves I’d prefer, but the other elements of the design to benefit sound quality to my ears with my experience are all met. 1/ The VA cathode resistor is unbypassed, which to my ear is the best sounding option when the choices are capacitor bypassed auto bias, battery biased, un-bypassed resistor auto biased. There is no coupling cap. The output stage is autobias bypassed with paper in oil. My preferred capacitor type in this position.

So the maths has chosen the valve and the preferences have accomodated the choice of valve. I expect the result good. Any other choice for this topology would compromise power out / sensitivity and frequency response through the interstage transformer, which is the elephant in the room I accidentally ignored.

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Just going off memory the power out is 7.5 watts at 5% second harmonic distortion. When the parts arrive and its built I’ll review to sound.
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#26 Re: PP3521 idht

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#27 Re: PP3521 idht

Post by ACPEN »

Await resultswith interest- never tried doubling up with PP3521.
I see Langrex still have some ( unless you've just bought them up!)
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#28 Re: PP3521 idht

Post by Paul Barker »

No O havent bought any more since I bought these many years ago. Th heater voltage is a resounding success 34 and a bit volts.

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#29 Re: PP3521 idht

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I couldnt find any E88CC’s or ECC88’s in the house. Ill have a bag at the lockup a few miles away but ive got a scope full in the workshop so no worries. I’ll use two opposite way round so they last twice as long. Found a shocking amount if d3a’s looking though!
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Last edited by Paul Barker on Sat Jan 13, 2024 7:34 pm, edited 1 time in total.
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#30 Re: PP3521 idht

Post by ACPEN »

How do you power them? I've used ac from a transformer or full wave dc doubler from 7.5v transformer. Neither caused hum problems.
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