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Old 22nd September 2012, 01:16 PM   #11
rrrs is offline rrrs  United States
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Hi, thanks, yes, that is what I was worried about.
Ongaku is using 6072 (12AY7) in SRPP configuration followed by a 5687 cathode follower driving a 211.
TriodeDick got a simple Mu Stage in his 2-stage 211 amp. Looks that would be easy enough to implement as I got needed voltages available.
Would need to chenge my input tube from e88CC to 27 and add transformer to suply 2.5 V filament for 27.
I alredy got some D3A tubes left from phono amp project, so that is a bonus.
If someone has other sugestion that could work fine, please let me know.
In particular, it would be great if I could keep existing 9 pin sockets...
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Old 22nd September 2012, 01:18 PM   #12
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With such a high ht why run the e88cc @90v with a load resistor? perhaps a good upgrade would be to lower the value of ps dropping resister and raise the load resister? or even use a ccs.
Or even better what rrrs said...
enzo
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Old 22nd September 2012, 01:20 PM   #13
rrrs is offline rrrs  United States
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I forgot to post the shematic:
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Old 22nd September 2012, 11:39 PM   #14
kevinkr is offline kevinkr  United States
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A 76 would slot right into that TriodeDick circuit. My experience with the 27 has been variable - there are some good ones but an awful lot of them seem to be microphonic. The 56 is often regarded as being interchangeable with the 27 but actually has significantly higher mu. The major difference between the 56 and 76 is filament voltage, the 56 being 2.5V, and the 76 being 6.3V.

Note that the 27 has a mu of 9 vs mu of 13.8 for the 56/76.

I also recommend the 6C5 and 6J5 which with a mu of 20 should give you full output at a little under 2Vrms in.. Consider the 6FQ7/6CG7 if you want to stay with a 9 pin socket.
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Old 23rd September 2012, 01:39 AM   #15
rrrs is offline rrrs  United States
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Hi Kevin, thanks a lot for sugestion, 6FQ7/6CG7 could work particulary well as I could stick to my alredy available two 9-pin sockets and just rewire them from my current circuit to TriodeDick Mu stage to convert curent 3 stage into 2 stage amp.
Other option will be to keep my 3 stages and reduce the voltage drop across my 5687 SRPP second stage in order to stay within Max heater to cahode voltage rating; I could get close to 400v drop across the SRPP (200v per section) that way; surently there is 575V drop. at the same time I could reduce the Rka and Rk resistors in SRPP to increse the current; with present 4.7K it is drawing only just above 3mA. Would going uo to usually sugested 10mA for 5687 tube be right and help keep the headroom?
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Old 23rd September 2012, 02:04 AM   #16
kevinkr is offline kevinkr  United States
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I'd go for the TriodeDick circuit personally, the lower level of complexity I'd expect to pay you back with better resolution.
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Old 23rd September 2012, 05:09 AM   #17
rrrs is offline rrrs  United States
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Yes, makes sense, going to 2 stage should be improvement, just would like to chek on few practical considerations:
Since there is aproximately 230V petential difference between two cathodes, I would probably hawe to add additional 6.3V transformer to stay within max heater-to-cathode voltage spec; in theory, if I use 6FQ7/6CG7 and elevate the heaters to 110V I would be RIGHT on the limit of max spec (100V above 6FQ6 cathode and 120V below D3a cathode), but I am not sure it is a good idea to be that close to the max. rating?
Looking at tube data it seems that 5687 could also work instead of 6FQ7; it has mu of 17. Reason for considering 5687 is that also fits 9 pin socket and I alredy got some tubes.
I noticed TriodeDick uses some pretty large capacitors (0.33uf, 1 uf and 2.2uf cupling capacitor) vs my current circuit using 0.1uf and 0.47uf; was wondring if it is really necessary to use this high values; would be nice to be able to use what I got. I am using my amp just above the 70Hz, so low end is not extension is not really a priority.
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Old 23rd September 2012, 07:34 AM   #18
rrrs is offline rrrs  United States
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Hi enzoastro, just wanted to clarify your comment re. the e88cc plate voltage; this is actually a mistake on the schematics, 90v is actually measured at e88cc plate, there is approximately 115V on top of plate resistor. e88cc draws approximately 3mA current at idle.
Anyway, I guess I will be trying to adopt TriodeDick circuit, as suggested by Kevin, so my input stage would not be used.
If someone would have some more suggestions of adapting TriodeDick circuit to my amp would be great to hear; especially it would be great if I could make use of tubes I got on hand (e88cc, 5687) instead of changing the socket and looking for 27.
Thanks for any suggestions!
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Old 23rd September 2012, 06:52 PM   #19
disco is offline disco  Netherlands
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Quote:
Originally Posted by M Gregg View Post
Or float,

with a 0.1uf polypropylene 600v from 0 to gnd.
The divider is better sometimes floating can overcome other problems.


Regards
M. Gregg
Which problems can be overcome by floating the heaters?
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Old 16th November 2012, 08:05 PM   #20
rrrs is offline rrrs  United States
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Default IT version

Hi Kevin, now, for some time I been using Triodedick circuit, but with 5687/D3a configuration. Was really please with performance, a clear improvement from my original 3 stage, SRPP driver configuration! Gain was JUST enough with my passive TVC as I got really sensitive speakers (around 100db).

Triodedick also suggested trying his latest, IT variation of the circuit, using IT instead of Mu-stage (see atached).
I got LL 1692/18ma transformer and am looking to try testing following tubes (got most of them already):
5687 at around 17mA/190Vp (8v bias)
5687 in parallel at around 10mA each/180Vp (8v bias) - (IT can handle up to 21mA if configured as 3.5 : 4
WE396a in parallel at 8mA each/150Vp (2v bias)
C3g (as triode) at around 14ma/190Vp (3v bias)
7788 (as triode) at around 18mA/190Vp (3v bias)
D3a (as triode) at around 20mA/160Vp (1.5v bias) here I could use IT as a 3.5:2 step-down to further reduce impedance, but am worried about low bias that might result in input overload.
Both, my Phono and DAC sources are spec to output around 0.7 Vrms, so in theory a 3V bias on the input valve would give me 6db headroom; but not sure if that is a really necessary consideration?

I noticed two ways for connecting pentode as a triode:
1) g2 to anode and g3 to cathode
2) g2 and g3 to anode
Is one of those "right" or "better" or is it application dependant; what is the difference in performance?

Thanks for your comments, also any thoughts on the choice/application of drivers I am going to test?
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