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Old 18th April 2005, 08:35 AM   #1
Yvesm is offline Yvesm  France
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Default Another 845 SE

Hi there !

Here are some thoughts :

For now, just the driver (V1, V2) was prototyped and showed good measuring results.
Roughly, near 400v pp with less than 1% THD.

Screen voltage of the EF184 seems to be critical, more investigation needed.

Changing the choke for a small OPT gives a good sounding tiny amp !

I've never used the 845 yet and I'm open to any suggestions.

Yves.
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Old 18th April 2005, 02:50 PM   #2
Radames is offline Radames  United States
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Default Why 400pp

Salut Yves,

Why do you need a 400V pp swing on the grid? With a 7K loadline, if my calculations are right, around 260V pp swing is the max possible. Limiting the swing to 220V pp would be probably best to reduce distortion, as with 260 you go into the curved parts of the Ep vs Ip plot.

A 10K OPT and a higher PS (1.1 - 1.2KV DC) would give you a bit more swing but not too much (280 pp or so). It will however drop a bit the THD at the output.

Just some thoughts on the output stage.

Anyway, at lower than 1% THD, it can be a fine driver. I cannot comment too much on the driver as I haven't started modelling pentodes yet.

Cheers,

Radames
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Old 18th April 2005, 04:06 PM   #3
Yvesm is offline Yvesm  France
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Default Why not !

Quote:
Originally posted by Radames
Salut Yves,

Why do you need a 400V pp swing on the grid? With a 7K loadline, if my calculations are right, around 260V pp swing is the max possible. Limiting the swing to 220V pp would be probably best to reduce distortion, as with 260 you go into the curved parts of the Ep vs Ip plot.
I agree, I meant that the driver is ABLE to give such level at 1% THD, better if there is some remaining headroom

Quote:
A 10K OPT and a higher PS (1.1 - 1.2KV DC) would give you a bit more swing but not too much (280 pp or so). It will however drop a bit the THD at the output.
I'm still undecided about the voltage and the load to use.
Too low, more distortion, too hi, lower bandwith for the same complexity in OPT interleaving !
Since I plan to wind the tranny, I've a tendency to stay on the lo side !


Thanks for this points
Cheers,

Yves.
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Old 18th April 2005, 05:33 PM   #4
Radames is offline Radames  United States
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Default Now a question

I was thinking in designing an 845 amp (SE). But as I wanted to keep it two-stage, I switched to the 211. Do you think that you could use a two-stage topology? Have you thought about it? If so, which input valves were you considering?

Cheers,

Rada
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Old 19th April 2005, 12:28 AM   #5
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G'day Yves,
I have been looking at the 845 for a while to see if there were any minor mods I could do to my Chinese "Music Angel" 845.
The popular wisdom is that something "special" happens with 845s when you get the Vak above about 1050V. That is: it allows the use of the best operating point to suit the tube. The top of the line 845s out there seem to use anywhere between 1100 and 1250 Volts - My Chinese amp uses 880V and I think that that is is it major limitation. So my advice is give the 845 plenty of volts - they seem to like it.
One other thing to watch is that modern production 845s don't seem to have the full 100W Anode dissipation capability - you might be wise to work on 75W Anode dissipation as a design point. So see if you can get the volts up another 100 to 150 volts to say 1150V and drop the current to about 65mA. If you are looking for Output Iron then Lundahl have a relatively new and highly regarded tranny to suit 845.
Cheers,
Ian
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Old 19th April 2005, 01:02 AM   #6
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Yves,
The Lundahl Output Iron is their LL1688. Designed to operate at 70mA idle with 9K2 Impedance. Rated at 30W.
Even if you go ahead with winding your own iron its worth a look to see how they did theirs - ie interleaves, core size, gap etc.
Cheers,
Ian
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Old 23rd April 2005, 08:20 AM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally posted by gingertube
Yves,
The Lundahl Output Iron is their LL1688. Designed to operate at 70mA idle with 9K2 Impedance. Rated at 30W.
Even if you go ahead with winding your own iron its worth a look to see how they did theirs - ie interleaves, core size, gap etc.
Cheers,
Ian
Downloaded the data sheet this morning !
Good informations.
Winding the tranny myself is a part of the final challenge of course !

Thanks, Yves.
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Old 23rd April 2005, 08:55 AM   #8
Yvesm is offline Yvesm  France
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Default Re: Now a question

Quote:
Originally posted by Radames
I was thinking in designing an 845 amp (SE). But as I wanted to keep it two-stage, I switched to the 211. Do you think that you could use a two-stage topology? Have you thought about it? If so, which input valves were you considering?

Cheers,

Rada
Roughtly, the 845 at 1000v needs some 300vpp grid drive.
Assuming you have a source giving 2vpp, you need a voltage gain of about 150.
This could only be attained with a small penthode but will probably exhibit too much output impedance to "shack" the 845 grid efficiently.

Could an interstage tranny be of some help

Or perhaps a two tube design, for exemple using a triode + penthode combo e.g. 6BM8, 6GW8 ...

Yves.
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