DIY Audio Projects Forum

Mini Block class-A PP EL84 Amp project
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Author:  famousmockingbird [ 04 Aug 2016, 11:10 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

wdecho wrote:
"you'll find the 6P15P-EV's to be unparalleled in speed and detail. The reason for this is because the 6P15P-EV is actually a video tube as opposed to an audio tube. This gives it much wider bandwidth than a normal audio tube and that results in great SPEED."

Is this Roger talking? Anyway it sounds like something he would say because it doesn't really make any sense because he is using it as a triode; i.e. The increased interelectrode capacitance and resultant Miller effect will kill the supposed bandwidth advantage. Also we are using these tubes for audio why would we care about bandwidth?

Lol whatever you say Roger :up:

Author:  wdecho [ 04 Aug 2016, 12:18 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

No it was Steve Deckert who made the comment on the 6P15P-EV tubes.

I did install a .1uf poly from the cathode to the suppressor grid per a friend of mines advice, Maynard. On the schematic Steve publishes for diy'ers this is not shown but he does discuss doing so in some of his articles. He does not specify the value though so Maynard and I took an educated guess. Does it help, maybe.

Author:  famousmockingbird [ 04 Aug 2016, 12:38 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Ah yes the "Hazen mod".

Although I am leery of this having any added benefit I shouldn't shoot it down before trying it. I suppose I could do it to my 6HJ5 amp.

Have you measure DC conditions before and after to see if the quiescent OP state is the same?

Author:  wdecho [ 04 Aug 2016, 14:22 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

I have not measured any differences, only listened to before and after. I want to think it did help. Could be just in my head and tubes just started getting better but it did take a number of hours on these tubes for the sound stage to really come around on both builds using these 6P15P-EV tubes. I am looking forward to hearing about what Bruce has to say after testing and listening to them for a period of time. Right now these tubes are not that well known about for audio applications vs the EL84's or 6P14P tubes others seem to like in EL84 applications. The problem for me with the 6P14P-EV tubes are they are around the same cost of EL84's.

Author:  famousmockingbird [ 04 Aug 2016, 15:13 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

I have used the 6P14P-EV with good success in vintage gear calling for 7189.

I like to think there are plenty of good cheap tubes still in the U.S.A.

These are only $5 each for NOS NIB. ... 206BZ8.htm

Author:  gofar99 [ 14 Aug 2016, 12:00 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi Everyone, My batch of 6P15P-EV tubes came in and I modded my pair of minis to use them. Just connect pin 3 to 1 or 6 on each output tube. I ran a batch of tests to see how they compare to other common tubes. The results are below. I used my HP distortion analyzer (it lumps noise and distortion together BTW), Tenma Signal generator (residual is 0.10 %), my Velleman PC based DSO*. Tubes were balanced to within 1% at idle and warmed up for about 30 minutes.

In Amp "A" 8 ohm load, 1 wrms into non inductive load.

Tube Type                     %Distortion @ 100HZ ,             @1000HZ                @10000HZ

EH EL84                            0.35,                                          0.34,                           0.41,
New Mullard EL84              0.57,                                          0.61,                           0.60,
JJ EL84                             0.33,                                          0.35,                           0.34,
6P15P-EV                         0.18,                                          0.20,                           0.22,

In Amp "B"
6P15P-EV                      0.20,                                             0.16,                           0.23

Frequency response was plotted at 20% steps from 10HZ to 30K at 1 wrms and 8 ohms.
It was +0/-0.3 dbv from 20HZ to 25KHZ. at 10HZ it was -0.7 dbv and at 30K it was -0.5 dbv. See the plot ...everything should look that nice. Output power was right a 5 wrms. I didn't measure the distortion there as the wave forms were still clean. Anomolies start to show at 5.9 wrms and go south from there. This is similar to the EL84 applications and is caused by the forced class A operation (by the CCS).

Amp A with 6P15PEV 1W 8R 0-30K.png

I spent a few hours listening to the amps on my main system. Very nice clean sound (you would expect that from the data). I personally did not detect any great changes. But since the 6P15P-EV tubes are dirt cheap and the results excellent it is certainly worth considering doing. I'm not about to undo the mod, but you could have the best of both worlds with a DPDT switch that would permit using both types of tubes. The EL84s have internal connections on pins 1 and 6 and the 6P15Ps use 1 and 6 for the suppressor grid (which is not connected to the cathode on pin 3).

*I should indicate that with a source of 0.1% distortion and a noise floor at about -80dbv the distortion levels are actually going to be lower. The actual relative values will be approximately the same though. What this means is that at the 1 wrms level these amps have extremely low distortion for a tube amp. This is particularly true based on their simplicity. If your system is efficient enough to require only a few watts these amps are killers.

Good listening

Author:  wdecho [ 14 Aug 2016, 20:06 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Thanks for doing this Bruce. It confirms what my ears are hearing. Is there any need to have a switch when you change from a 6P15P-EV to a EL84 since the EL84 suppressor grid is already internally tied to the cathode anyways. Does it hurt anything to also have the tube pins connected at #3 and #6 if they are already internally connected. I was thinking that nothing needs to be done to replace the 6P15P-EV tubes with EL84 for comparison. Your last sentence also confirms what I am hearing with my very efficient 103db horn speakers that rarely use more than 1 or 2 watts. I have been enjoying this amp very much. Very dynamic sounding. My 300B SET sounds a lot like it but with a slightly bigger sound stage but this PP amp has a very good sound stage much like some of the other SET's I have and probably does better on the bass. It comes down more to nitpicking than anything else.

Getting back to the 6P15P-EV tubes the soundstage does get better with some hours on them. They are great deal on a quality audio tube at the present time until the Russian NOS is depleted. Get some and stash them away.

Author:  gofar99 [ 14 Aug 2016, 21:32 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi, It was an inexpensive and easy to do experiment. Since my shop is set up to do the tests easily it was only a few hours to do. As for leaving the jumper im place...I don't know for sure. The tube specs for EL84s indicate that pins 1,6 and 8 are used for internal connections (not necessarily to each other though). Some might have been to the getter and others support pins for some of the main elements. None of the tubes I have here seem to have anything attached to those pins. It is possible that they are support struts and I would be wary of internal acing. I used pin 6 for the connection in the 6P15P-EVs. If you leave the jumper in place the worst that can happen is smoke, fire, arcs, flashes and loud noises...followed quickly by destruction of your speakers if attached at the time. I actually suspect none of the above will occur at the voltages used in the Poddwatt and it is likely safe. I plan on leaving the amps in the system for a while and seeing (hearing) how they perform. At least until I need some muscle and then the KT120 amps go back in. BTW any of the Oddwatts can handle difficult loads. They all play nice with my Martin Logan ESLs and they are about as difficult as you can get. Wild crossover (a zillion components and it is asymmetrical) and the ESL part drops from just over 4 ohms at 400HZ to 1.2 at 20K. A difficult load for a tube amp.

Good listening

Author:  wdecho [ 15 Aug 2016, 06:36 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

If and when I buy some EL84's I might be be brave and give it a shot. If the suppressor grid is internally tied to the cathode as the specs show it should not cause a problem being tied together at the pins as well, at least from my limited understanding. I have bigger and more powerful Class A amps as well and I am like you and like this amp because it does not heat up the house like the 300B I have and sounds plenty good enough. I also agree it acts like it is much more powerful than it is. It is clean, and quick with a great soundstage and can be bought as a complete kit with high quality components.

Author:  famousmockingbird [ 15 Aug 2016, 11:10 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

It looks like wdecho found some really nice valves for the miniblocks.

Maybe it raises some questions.

Looking at the datasheets we know that the internal impedance of the 6P15P is more than double that of the EL84. 38k to 100k. Transconductance is also higher at 15mA/V compared to 11mA/V.

Some of my reading material while on the thrown is Mullard's "Tube Circuits for Audio Amplifiers". When running ultra linear in the 10 watt amp they are using a 6k6 output load for 20% screen taps and 8k for 43%. The output iron for the miniblocks from Edcor advertise as 10k and 40% screen taps.. Mind you Mullard also was running higher anode voltage compared to the miniblocks and the load they chose is still lower. Edcor has a 10w 8k or maybe the 7.6k 25w are worth trying. Maybe Bruce will clue us in on why he chose 10k. I mean TBH the numbers he gets are just fine so I am not trying to say anything negative, just observing. Another possibility is modern EL84s are not the same or as good as vintage specimens. Have their been distortion measurements using some NOS tubes?

Either way the 10k load really works well with the 6P15P tubes, great find!

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