RH300B

Alex Kitic · Post by **Alex Kitic** » Sun Nov 17, 2013 11:37 am

I have finally published the long awaited RH300B amplifier (RH amplifier with 300B output tubes) on my blog.

All those interested in 300B amps might find it a good project, and certainly a good read...

Paul Barker · Post by **Paul Barker** » Sun Nov 17, 2013 1:59 pm

Thank you.

Alex Kitic · Post by **Alex Kitic** » Fri Nov 22, 2013 7:54 am

This silence is refreshing, but I'm not sure whether it is due to a lack of interest, or a lack of words?

At least no-one is complaining about the triode driver (because they read in a reputable book that predates computer assisted design that the pentode driver is the way to go).

I guess I've gone too far this time.

Cressy Snr · Post by **Cressy Snr** » Fri Nov 22, 2013 9:10 am

Well, I certainly have no complaints about the ECC81 driver in my amp.
Neither do any of the guitar amp boys.
I was initially a bit dubious about using 9 pin miniatures of the ECC** variety but the 81 in particular has a useful combination of highish gain and driving capability that makes it a good choice for driving output pentodes.

You are not going to find many dissenters on here, as this place was created as a forum for people who were fed up as Nick says of having to justify every damned resistor to armchair engineers, who want to be awkward for the sake of it.

Also the forum tends to go a bit quiet after we have had one of our twice yearly togethers at Owston.

Dave the bass · Post by **Dave the bass** » Fri Nov 22, 2013 11:02 am

Alex Kitic wrote:This silence is refreshing, but I'm not sure whether it is due to a lack of interest, or a lack of words?

Thats cos you're amongst friends here, not followers. You haven't re-invented the wheel but you have made a nice wheel!

I do that lots too.

You've got your way of doing things, other's have their way's. That's accepted here, thats probably why as our-Steve said you haven't been 'dissed'.

Keep it up (everyone!).

DTB

JamesD · Post by **JamesD** » Fri Nov 22, 2013 11:07 am

Alex,

If you have read the forum you will know that we tend to evolve a design infront of our peers and discuss it whilst it going on and then debate how it sounds when we hear it either at home or at Owston.

presenting a complete design that is proven and finished is always interesting and appreciated but is also a bit underwhelming in terms of participation - so the forum response is likely to be muted until someone builds it and posts the process here. Its not criticism or disinterest - its just not something we tend to respond to much.

Now when someone does build it we'll all chip in with comments

On the triode driving the 300B in partial feedback configuration - well we've done that years ago and even used an Aikido driver in partial feedback as the driver so we know about Schade Feedback from a low impedance driver and how it works - Dave Doves amplifier was built this way and shown at EggFest back in 2006 and was very very good- Damn that was before this forum and on the old WAD forum...

We have also worked through T-Rex amplifier (from John Broskie and Stephie Bench) another 300B driven by triodes in partial feedback configuration so no surprise it works (OK thats a cascode double triode):D

Keep up the good work though - you have a lot of people who appreciate what you are publishing!

J

pre65 · Post by **pre65** » Fri Nov 22, 2013 11:14 am

And don't forget the "monkey" 300b variations that some of us tried.

Dave the bass · Post by **Dave the bass** » Fri Nov 22, 2013 11:18 am

Aaaarrrrrggggghhhh! My RF oscillator!

DIET, that was 2A3 Munkey, not 300B. Doh!

DTB

JamesD · Post by **JamesD** » Fri Nov 22, 2013 1:31 pm

After reading about the RH designs on other forums, I've come up with a criticism of the triode driver

Its ridiculous to use a highly linear output tube - the 300B and such an obviously non-linear driver - I mean have you seen the curves for that driver? They are miles away from linear spacing!!!

And then to be so wrong as to put a feedback loop around the linear tube and leave the non-linear driver tube outside the loop

Crazy, just crazy!!!

What sort of person designs like that? It so obvious he doesn't know what he's doing and he must be blind 'cos he can't even read Schade's patent and learn how it should be done!!! I'm staggered that he has the nerve to post such a thing - particularly on a forum like this!!!

Well, I'm speechless!!!!

Alex, hope that makes you feel better about posting here

James

Dave the bass · Post by **Dave the bass** » Fri Nov 22, 2013 1:44 pm

^^^^ *grabs tub of Popcorn and awaits scrap...*

DTB

Ali Tait · Post by **Ali Tait** » Fri Nov 22, 2013 3:11 pm

Tee Hee.

Alex Kitic · Post by **Alex Kitic** » Fri Nov 22, 2013 3:31 pm

What can I say, thanks for the support!

The original RH300B dates somewhere in 2005, and has never been built as such, since I had no 300B tubes to build it, and it seems that no-one wanted to build it before the project was "finalized".

Maybe some have built in the meantime, and kept silent about it, or just passed it as an interesting amp of their own...

When I begun finalizing it (once I had the tubes), I wanted to try fixed bias... thus the main difference between 2005 and 2013 is the cathode follower tube... which is not necessary, but helps keep distortions lower and output power higher than with the old schematics.

As for the anathema triode driver, it is actually inside the feedback loop

The T-Rex is indeed interesting, but much more "complicated" (I like keeping it as simple as possible)... and I have not been able to trace the Dave Doves 300B amplifier schematics, so I cannot compare.

If anyone would like to try it, I'll be glad to help with advice and suggestions, as usual.

PS
I am far away from you, and where I live there are no relevant DIY fests I could participate... and I was not aware that you develop projects as a group (not much of a team player, frankly).

Cressy Snr · Post by **Cressy Snr** » Fri Nov 22, 2013 4:01 pm

Alex Kitic wrote:What can I say, thanks for the support!

I am far away from you, and where I live there are no relevant DIY fests I could participate... and I was not aware that you develop projects as a group (not much of a team player, frankly).

There are people on here who plough their own furrow such as Paul, Nick and JamesD. These guys are highly experienced, but always give their valuable advice freely and without prejudice to others like me who are competent designers,but not at their level.
That's what makes this forum such a great place to be. Members get advice and help with their projects but never get talked down to or made to feel like an idiot.
Makes for good relationships and friendliness all round.

IslandPink · Post by **IslandPink** » Fri Nov 22, 2013 4:50 pm

James - you're can't pull off that DIY Audio-stylee convincingly : everybody here knows you've actually designed and built things yourself

Alex - have you run the 300B with & without the LM317 current sink ( I mean, with normal cathode bias perhaps ) ? I say this because the 317 used in this way has a reputation for adding quite a bit of 3rd and 5th harmonic .
I remember Allen Wright had a LM317 current sink in his 300B PP amps, that I saw & heard at Euro Trode Fest a few years ago - and the tone was rather sharp and aggressive to my ears and a few others who heard them .

Alex Kitic · Post by **Alex Kitic** » Sat Nov 23, 2013 4:57 am

IslandPink wrote: Alex - have you run the 300B with & without the LM317 current sink ( I mean, with normal cathode bias perhaps ) ? I say this because the 317 used in this way has a reputation for adding quite a bit of 3rd and 5th harmonic .
I remember Allen Wright had a LM317 current sink in his 300B PP amps, that I saw & heard at Euro Trode Fest a few years ago - and the tone was rather sharp and aggressive to my ears and a few others who heard them .

I have run the RH300B only in the configuration shown. I have stopped building with cathode resistors a long time ago. This is the configuration I use in all other RH amps, besides the added resistor with a passive resistive role. Besides protecting the LM317 from high voltage at the input, it dissipates extra heat, making life easier for the regulator. Of course, voltage drop across the resistor is always the same, regardless of the tube type or condition, thus the amp is safe up to 75V of bias, which cannot be achieved by any 300B tubeat this current with this power supply, regardless of compatible rectifier used. I call it "disaster-proof", just sit and enjoy and roll tubes, since every slight change you make will be audible.

Simulations do not show an increase in odd harmonics, and it certainly is not audible as such. The tone is neither sharp nor agressive, although there is none of the syrupy sound some seem to like. It sounds perfectly in line with the other RH amps, but with accents givenby the output tube used, 300B (and the sound changes between "mesh" and EH300B, as expected.

I do not know the amps you mentioned, but as you said, they're PP, while the RH300B is an SE amp. Probably the amps you did not like used one un-bypassed LM317 for both output tubes, which sounds different than bypassed, or than one LM317 per tube with bypass.

audio-talk

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