DIY Audio Projects Forum

I forgot to add the driver is swinging much less to get to full output power with the 6P15P, this could contribute to the reduction in distortion too.

Hummm. My testing indicated the opposite. I had to increase the signal to get the same level on the scope and distortion analyzer. Mine use NOS Philips 5751s. Perhaps a different batch of 6P15Ps

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Hi Bruce, where does the bias on the output land Vg-k~ -5 ?

What I think is going on is since the 6P15P has more gain you end up with a larger negative feedback signal at the cathode of the SRPP which is why you require more input amplitude.

But to drive the 6P15P to clipping should only require 10v peak to peak @230v 40mA.

I decided to replace the polyester coupling caps with my go to cap, orange drops. They are the best bang for the buck caps I have tried. From listening to one of my favorite LP's I definitely see an improvement. Vocals are much better and smoother. I am sure the ones Bruce suggest would be even a better choice. For those that do not want to tie the #3 and #6 pins of their amps this is an alternative that others have done to be able to use 6P15P-EV's in an EL84 amp.

http://s244.photobucket.com/user/the_fr ... m.jpg.html

Not hard to do and there would be no modifications to your present setup. An excellent NOS surplus tube for audio use. At the prices NOS tubes bring now these are one of the best deals out there.

Hi, V(GK) runs in the 5-6 volt range. With the mod to add some positive voltage (between 5-8 volts) to the grids to raise the cathodes up by a like amount to get over the kink that happens when the LM317 gets driven too low the voltage on the grid to ground can range from 10-14 volts typically it is about 10. The drive however remains at + and - 5 volts (10 total) as that is all that is needed to drive the tubes full on or off. It is an easy chore for the 5751.

I put Audyn True Copper Caps in mine. Awesome sound but at a price considerably over the PIOs in the project. You need both great speakers and really good ears IMO to really appreciate the difference though. I recommend the less costly caps for most uses.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

I put Audyn True Copper Caps in mine. Awesome sound but at a price considerably over the PIOs in the project. You need both great speakers and really good ears IMO to really appreciate the difference though. I recommend the less costly caps for most uses.

Good listening
Bruce[/quote]

I had sticker shock when I priced the Audyn's especially for someone like me that builds as a hobby. As soon as I finish one build I start planning on what to build next. For someone that is just building for one good amplifier it certainly pays to go for the best the first time and listen to what the designer says. I do not want to hear how good it might sound with Audyn's, it would do harm to my building budget. I am sure there are other best for the buck caps out there but for me orange drops are hard to beat for around $5 apiece.

Are you still liking the 6P15P's Bruce after getting some hours on them?

Hi, Yes the cost of parts can really open your eyes. I can get away with some costly parts as I have backing of an audio company (that coincidently I own half of). If you look at the projects I post and nearly all my recommendations they are based on less lofty components. One of my original and ever present current mantras is to develop projects that excell without being overly expensive. IMO there really is no cheap tube gear, but equally it need not force the builder to take out a bank loan. I do however indicate where I feel upgraded components might be beneficial. Even then I consider the gain vs cost. A typical example would be in the KT120 mono blocks. I would expect that one could be built for about $350-400 if the builder was frugal. This would still include the specified transformers (a major cost). But to upgrade the capacitors (2) to the Audyns would only add about $50 to the cost. IMO not a cheap amp, but certainly one that plays well above its price range. (I have been told way, way above many times). So it is a good cost to benefit arrangement. It becomes more fuzzy in things like the Poddwatt Dual Mono BLock and Mini Mono Blocks. I figure there it depends a lot on what you are going to use them for. If you should have some reasonably efficient full range high quality speakers and do serious listening, then yes it makes sense. If all you are going to do is background music in an office, not worth it.

I am still using the 6P15PEV tubes. They have a nice clean sound and play well with my Martin Logan ESLs. 5 watts per channel would not seem enough to make the ESLs sing, but the Mini Blocks (and Poddwatts) always have. Do I hear anything startlingly different, no. Just a tad more detail it seems and a little better clarity with upper mids. Could I tell in an A-B test which was which , dubious. They certainly are no worse than the EL84s and the bench tests indicate they are slightly better. They are certainly a good choice in the amps both from cost and performance aspects. My only concern is about the possibility of variable quality of them from different sources. Mine (8) were well matched and all the same date codes and markings. Others might not be. So while I certainly recommend them, I do so with some caution. BTW, I feel pretty much the same for new production tubes. I usually get mine from a very few reliable sources (same for the company mostly) as I have had less than satisfactory tubes from some other well known sources.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Roger Modjeski gave a speech at the Burning Amp festival and in the speech he talks on tube runs and how they were made from his visits to factories years ago. Very entertaining speech for those that are interested. I got a laugh from him talking about a European Co that found if they let the mix of cathode paste sit overnight at the factory is was better and they did not know why. Maybe the nightwatchman urinating in the batch is what made it better Roger said. It got a laugh from everyone. I very much enjoyed listening to Roger talk about tube circuits, of course sprinkled with his opinions being a player of tube equipment. From his description of runs they are always going to be differences in a run of tubes and especially from one batch to another. While matching is not always necessary it is probably best especially in PP amplifiers. My thinking is that the 6P15P-EV tubes being used for Military use should be as good as any run of tubes from that era.

https://www.youtube.com/watch?v=io_yZENYlxY

I am thankful for Bruce's testing and his opinions of these tubes because I have never heard an EL84 tube and I was curious on how these tubes compare. From reading information on the web the Russian 6P14P is liked very much by some for an EL84 replacement but their cost is in line with the common EL84 offerings. I am always looking for bargains and deals on tubes.

Hi Bruce, I should have been more specific, here I will break my thoughts down more clearly.

Here is what I calculate for gain for each stage:

"SRPP" ~48

output stage (EL84) ~22

output stage (6P15P) ~28

10k:8 OPT ~ -35.3

Your feedback resistors are 150 and 3k3 giving us a feedback factor of .043 150/(150+3300)=.043

With the EL84 open loop gain is roughly 30 and w/ feedback it's 30/1+(30*.043)=13

With the 6P15P open loop gain is roughly 38 and w/ feedback it's 38/1+(38*.043)=14.4

Let's look at the feedack factor again: EL84 30*.043=1.29 6P15P 38*.043=1.63

The 6P15P is higher.

Because you have not adjusted the feedback factor of .043, the larger open loop gain from the 6P15P yields a larger voltage at the cathode of the first gain stage, since the input of that stage is across the grid and cathode you deduct the now larger signal (compared to EL84) from the input. e.g. You need more input voltage to drive the 5751 for the same output of the that stage. The increase in negative feedback is probably why you get less distortion in the amp with 6P15P's.

The voltage from grid to cathode of the 6P15 should be about 1.5v less compared to the EL84, and since the 6P15P has more gain I was saying they require slightly less drive voltage for full output.

Hi all,

I am looking at building this project, but with a couple of small modifications. The main modification I had a question about was related to the use of the LM317 and the balancing pot. Rather than having an arrangement that balances the current between the two valves, I want to be able to adjust the current for each individual valve. After reading through this;

http://diyaudioprojects.com/Technical/V ... lators.htm

And this;

http://diyaudioprojects.com/Technical/C ... Regulator/

My understanding is that if I wanted to do this, I would omit the 25 ohm pot shown in the schematic, and instead replace the 15 ohm fixed resistor that goes from the output of the LM317 to ground with a pot instead - somewhere between 30 and 50 ohms.

I have four of these panel mounted ammeters that I was planning on using to monitor the current for each valve;

https://www.toroidal-transformer.com/sh ... 100ma.html

So the cathode of each valve would be connected to an ammeter, then from the ammeter to the input of the LM317, with a 50R pot connected from the output to ground to control the current - would that be correct?

Apologies if I have this all wrong, I am only a beginner, and whilst I have built amps from kits before, this is my first time building from a schematic, and I wanted to try and understand some parts of it better.

Any help would be greatly appreciated.