Change RTP3 to accept 6h30 or 6N6P tubes
Hi Jaap,
Have you really altered it and do actually use 6H30 or 6N6P in your RTP3?
What changes are required then?
In my opinion best position to use these would be for the cathode follower since a 6922 might not be the best tube around for that purpose.
Additionally one may use them in the line stage cascode however both tubes have lower µ compared to the 6922 and as such I would assume it doesn't make much sense to use them in the phono amp as well because it will lower the phono gain or?
Another disadvantage is that both tubes are higher in size so they do not fit in the regular preamp case without drilling holes in the top cover.
The 5687 as a possible other alternative as was mentioned as well by Allen Wright will fit however. According to a threat on audioasylum he said that only heater current and some wiring needs to be adapted since the 5687 has a different pin layout compared to the 6922.
What is your opinion?
Thanks
Hi Jaap,
Have you really altered it and do actually use 6H30 or 6N6P in your RTP3?
What changes are required then?
In my opinion best position to use these would be for the cathode follower since a 6922 might not be the best tube around for that purpose.
Additionally one may use them in the line stage cascode however both tubes have lower µ compared to the 6922 and as such I would assume it doesn't make much sense to use them in the phono amp as well because it will lower the phono gain or?
Another disadvantage is that both tubes are higher in size so they do not fit in the regular preamp case without drilling holes in the top cover.
The 5687 as a possible other alternative as was mentioned as well by Allen Wright will fit however. According to a threat on audioasylum he said that only heater current and some wiring needs to be adapted since the 5687 has a different pin layout compared to the 6922.
What is your opinion?
Thanks
I built mine with completely isolated PSUs for the two channels. I did this mainly because I felt it was logically the best way to do the job, I had space in the PSU box, and mains transformers are not that expensive.
I have no idea whether the advantages are big enough to hear. My suspicion is that with a differential circuit like the RTP3 the influence of the PSU is minimised, so crosstalk through the HT and grid supplies should be negligible, particularly with a separate HT shunt regulator per channel.
As Zen_mod mentioned, I think the main potential advantage of dual PSUs is the potential to eliminate ground loops.
Alex
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I would definately consider a tube replacement for the first stage of the line amp. I used a ECC88 for the first stage on my first FVP5 build and it has way to much gain - even with a voltage divider over the volume pot. This valve will generate the dominant character of the amp as the second stage is virtually transparent.
The ECC88/6922 is absolutely the correct tube for the cathode follower as it was designed for this type of cascade arrangement. Stick with it as the best choice.
I built my second version using ECF80's and the gain arrangement is much better with the lower amplification at the front. I also configured the CCSink as a pentode for better performance.
Cannot see me ever having to build another preamp to replace it.
Shoog
The ECC88/6922 is absolutely the correct tube for the cathode follower as it was designed for this type of cascade arrangement. Stick with it as the best choice.
I built my second version using ECF80's and the gain arrangement is much better with the lower amplification at the front. I also configured the CCSink as a pentode for better performance.
Cannot see me ever having to build another preamp to replace it.
Shoog
I have not yet built the preamp but used the differential cascode as a driver in a PP amp.
this is what Allen Wright a few years ago said about using the 6H30:
=============================================
n Reply to: Re: Cascode (not cascade) the two 6H30 sections (nt) posted by dhsettim on July 2, 2003 at 21:57:58:
I am using the 6H30 as the input/driver tube in our about to be released PP-2CS poweramps (kit & finished).
It's a differential cascode topology as used in my differential preamps, and also the PP-1C schematic that can be found in the schematics section on my site. URL below. The output stage is the main change over the PP-1C, and is not being published until the next book comes out...
These are the operating points I use in differential mode - but the same points will be just fine for SE use as well - I don't find them too critical. Just don't run them at low currents.
With a B+ of 400 I place 75 volts on the lower tube's anode, and 250 on the upper tubes anode. This puts 150V across the anode load R (12k5) which equates to 12mA running current in *each side* of the diff.
This should give you a lot of gain, at least 100x, so you can use some UNBYPASSED cathode R's to bring it down to what you need and decrease the already very low distortion even further.
As I only use them in differential mode with the running current set by current sources - I can't suggest cathode bias R's for SE use.
One of the beauties of differential topology is that you can set the running current totally independantly to the gain without needing any cathode caps at all - while SE is often a trade off between gain & current.
Good hunting - it's a great sounding tube!
Allen
Allen Wright 01:58:35 02/18/04 (21)
In Reply to: A cascode 6H30 vs a 6CH6 pentode driver in a SE amp.....(long) posted by Anthony Dockrill on February 17, 2004 at 16:11:00:
Anthony,
Very pleased to hear that my belief in the sonic values of cascodes is not purely a personality defect on my part...and excited to hear that this works for SE as well as diff.
In the dpa300B amps they run at 85V on the lower anode, 250V on the upper and about 12mA per tube - in differential of course - and 12k5 anode loads (Mills).
What operating parameters did you come up with for your 6H30 cascode?
How much gain does it provide?
I'd be interested in seeing the whole schematic...
Allen
Shoog,
If the line stage in the RTP3 has too much gain it can easily be adapted by
changing two resistors in the line stage cascode and it can be lowered to much less than 10dB, so that alone wouldn't be reason enough for me to go over to another type of tube.
For me it would be more sonical reasons to change and a 6H30 might be better for cascode operation as Allen Wright said: "A tube for good cascode usage needs HIGH(!!!) transconductance, which eliminates ALL DHT tubes. The perfect tube would be the Russki 6S45, second the 6H30, 3rd the ECC88."
A 6S45 can't be used straight away in an existing RTP3 since it is a single and not a double triode, a 6H30 however is a double triode, so could be used and will give lower gain of course.
I'm not so sure if the 6922 is as you say absolutely the correct tube for the cathode follower. You are right that the 6922 is designed for cascode ( not cascade! ) operation however the cathode follower applied in the RTP3 is something completely different. The SLCF ( super linear cathode folower ) as it is called here is a normal cathode follower with a boostrap on top and a constant current source at the bottom and not a cascode.
One of the things a cathode follower should do is deliver current and in terms of that 6H30, 6N6 and also 5687 are better than a 6922.
Comparing the 5687 to the 6922 Diego Nardi a well known italian tube amp designer said the following in Sound Practices issue 10 about the tube of choice in his WOT ( with output transformer ) line amplifier: "The 5687 is superior to the 6922 in linearity and low signal level performance....."
And that is what makes me curious as well for a possible tube change.
airtangent
Do you know the values of the caps in the RTP3D PSU? If you use a choke regulated supply for the HT, as Allen recommends, you shouldn't need more than 47uF. In mine, with a 10H choke, I found that 2uF before the choke and 47uF after the choke gave me close to 350V to feed the shunt regulator. 47uF at 630V is easy to find - I used SCR polypropylene.
Alex
Allen didn't recommend line stage gains in the RTP3 below 10dB, as he found that it got too noisy:
I have tried both 10dB (PGS=15K) and 0dB (PGS=47K). Neither are noise optimum. Realise that the noise from this stage will always be the same - no matter what gain you set with the PGS resistor as this R only changes the differential gain and NOT the overal noise gain. To lower the gain AND lower the noise the same amount - you have to do the gain adjusting with the SGS, the one between the anodes of the cascode. But I only recommend that option in special situations as I find it doesn't sound as good as does using the PGS position.
Mine is set with the PGS at 10dB, and noise is not an issue. I haven't got around to trying any lower gains.
Alex
Alex M,
Ok, maybe that ( optimum noise level ) is the reason why Allen Wright supplied a 4k2 PGS resistor with my RTP3C kit years ago.
However a 20k SGS resistor was delivered as well although it wasn't shown to be used in the instructions.
But using a 4k2 PGS without a SGS resistor will result in 20dB gain which I consider a bit high.
Ok, when you say PGS set to 10dB with 15k ( without an SGS resistor I suppose ) noise is not an issue it should be fine.
Your remarks regarding the difference between lowering the gain with SGS or PGS are very interesting information for me as I was considering to try out later on maybe the extreme version of the RTP3.
Unfortunately I missed the chance to talk about that with Allen Wright.
airtangent
Despite the low Transconductance (which isn't an issue for the frontend stage) - I would seriously consider trying a DHT as the front end. Working into the high load of the second stage should make it sing nicely. All the benefits of a DHT tube amp, with all the grunt and drive of Indirectly heated high transconductance cathode follower.
Just my gut feeling, and if I was ever tempted to build an ultimate FVP5 - it would be this configuration.
My experience with the 5687 is it can sound slightly dark, and is quite sensitive to setup and hum pickup.
Shoog
Just my gut feeling, and if I was ever tempted to build an ultimate FVP5 - it would be this configuration.
My experience with the 5687 is it can sound slightly dark, and is quite sensitive to setup and hum pickup.
Shoog
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Shoog,
I know lots of people on the internet raving about DHT's and in a single ended
preamp design they might be worth a try although DHT's at least old types from the pre world war II era are not easy to get in good condition.
However this threat here is going about the RTP3 which is a balanced design and I can't remember ever having seen a balanced tube preamp build with DHT's.
I do not share your opinion that a 5687 sounds dark as I think it depends a lot on the complete stereo setup. My 5687 based WOT preamp doesn't sound dark as I feel.
Since 5687 tubes tend to leak a bit between cathode and heater according to Diego Nardi care must be taken with a good heater supply. A standard LM317
voltage regulator will probably not be good enough.
However when using DHT's that is true as well for their heater supply and lot of effort must be taken to get them hum free.
airtangent
The Amity and Raven power amps both use DHT in a balanced design - as does Gary Pimms 47 amp. So if there are benefits to be had - then they are applicable to all amplifiers. There is also of course the 300B amplifier produced by Allen Wright himself.
I have never seen much point in going that extra mile - as what I have is good enough - but its always an option.
As to the 5687, you have highlighted the issue with its susceptibility to hum pickup. Since I never consider it either necessary or desirable to use regulated heater supplies, then it makes the 5687 a problematic valve to use. I have used it though and it sounds good - though I think even the ECC88 can sound better if used correctly.
These are all just my opinions though - but as I have said before - I respect the opinions of Allen but don't take him as gospel.
Shoog
I have never seen much point in going that extra mile - as what I have is good enough - but its always an option.
As to the 5687, you have highlighted the issue with its susceptibility to hum pickup. Since I never consider it either necessary or desirable to use regulated heater supplies, then it makes the 5687 a problematic valve to use. I have used it though and it sounds good - though I think even the ECC88 can sound better if used correctly.
These are all just my opinions though - but as I have said before - I respect the opinions of Allen but don't take him as gospel.
Shoog
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yes
works fine and sounds better than all my nos 88 i have. i only changed the working horse in the middle of the slcf.
i´ve seen fotos from allens rtp with this tube in no idea if it´s worth to change all 3 tubes
happx x mas
works fine and sounds better than all my nos 88 i have. i only changed the working horse in the middle of the slcf.
i´ve seen fotos from allens rtp with this tube in no idea if it´s worth to change all 3 tubes
happx x mas