the Poor Man 660 support thread

[radiance]

here some 9V  10A toroids on the black market..

http://www.groupdiy.com/index.php?topic=38001.0

 

[Kingston]

I've been experimenting with 6K4P's all weekend.

I didn't want to end up with oscillation with some quick slap-on-wire-only tests, so I built a very sturdy adapter from each 9-pin socket to two 7-pin sockets using turretboard/bus-wire/sockets combo for each 6bc8 position.

I looks quite impressive actually, I should take some pictures.

Anyway, I was able to match these even better, quite perfectly actually, since I wasn't stuck with dual triode halves. I bumped B+ up to 165V and set the trimmers to 5,5V each (just a quick test, I'll optimise later). It allows about 22dB maximum gain reduction, and more headroom. Of course I had to experiment with more B+ for more headroom, but less maximum gain reduction. 190VB+ would allow around 15dB max gain reduction, for example (tested quickly)

I don't know why some people said earlier 6K4P is not a good fit for poorman, and why it was suggested only about 6dB gain reduction would be possible. 6K4P is a brilliant fit actually. It cuts the unusable distorted ranges available with the 6BC8, basically fixes gain staging. 6BC8 is simply not capable of 40dB gain reduction in this circuit, so why even have that much range available? It just sounds bad.

6K4P makes a much more balanced set up with it's wider grid curve and it's no longer easy to make poorman a distortion generator. VU meters now display much more usable information, instead of sitting "buried" whenever something compresses.

And the sound, obviously the the 6K4P at higher B+ compress a bit different than 6BC8. To me, this sounds more like the big brother and its variants, more controlled somehow.

 

[jackies]

Hey Kingston, great job!
How many 6K4Ps did you buy to get two sets of 4 matched?

 

[ChuckD]

Hello, can anyone comment on the use of this power transformer over the EDCOR version, which I have read is under spec'd
thank you, Stuart

I can .  The Edcor puts out terrible EMI interference which will couple into your T1 and then you are screwed.  At -55db you can get hum etc.
You could build a large Shield from Grain Orient Steel and Mu Metal and reduce this down to -85db ( I did)
But for a real fix either put your audio transformers in another location ... a few meters away! Or get the rondo from Volker. Worth every penny. No EMI at all.


Chuck

 

[Kingston]

Hey Kingston, great job!
How many 6K4Ps did you buy to get two sets of 4 matched?

Roughly every third or fourth on the rather large set I have is a match. 20-30 sets on ebay were below €20 for some reason, and these are the more tightly spec'd military versions (-EB). I guess these are a sleeper like the 6N5P.

There are a lot of matches because you no longer have to live with dual triodes, and can match pairs individually. I managed a perfectly matched octuplet! (if that's a word)

 

[lolo-m]

I don't know why some people said earlier 6K4P is not a good fit for poorman, and why it was suggested only about 6dB gain reduction would be possible. 6K4P is a brilliant fit actually. It cuts the unusable distorted ranges available with the 6BC8, basically fixes gain staging. 6BC8 is simply not capable of 40dB gain reduction in this circuit, so why even have that much range available? It just sounds bad.

6K4P makes a much more balanced set up with it's wider grid curve and it's no longer easy to make poorman a distortion generator. VU meters now display much more usable information, instead of sitting "buried" whenever something compresses.

And the sound, obviously the the 6K4P at higher B+ compress a bit different than 6BC8. To me, this sounds more like the big brother and its variants, more controlled somehow.

I did say that 6BA6 (and so 6K4P) is not suitable in the PoorMan. The grid voltage swing needed to have -10dB of effective gain reduction is far more than 12Volts.
How did you check the gain reduction ? With the Vumeter or with a real Gain meter ? Did you use the SCAB or stock Sidechain ?

But I agree on one point, the 6BA6 sounds cleaner than the 6BC8  .

 

[Silvas]

I've been experimenting with 6K4P's all weekend.

I didn't want to end up with oscillation with some quick slap-on-wire-only tests, so I built a very sturdy adapter from each 9-pin socket to two 7-pin sockets using turretboard/bus-wire/sockets combo for each 6bc8 position.

I looks quite impressive actually, I should take some pictures.

Anyway, I was able to match these even better, quite perfectly actually, since I wasn't stuck with dual triode halves. I bumped B+ up to 165V and set the trimmers to 5,5V each (just a quick test, I'll optimise later). It allows about 22dB maximum gain reduction, and more headroom. Of course I had to experiment with more B+ for more headroom, but less maximum gain reduction. 190VB+ would allow around 15dB max gain reduction, for example (tested quickly)

I don't know why some people said earlier 6K4P is not a good fit for poorman, and why it was suggested only about 6dB gain reduction would be possible. 6K4P is a brilliant fit actually. It cuts the unusable distorted ranges available with the 6BC8, basically fixes gain staging. 6BC8 is simply not capable of 40dB gain reduction in this circuit, so why even have that much range available? It just sounds bad.

6K4P makes a much more balanced set up with it's wider grid curve and it's no longer easy to make poorman a distortion generator. VU meters now display much more usable information, instead of sitting "buried" whenever something compresses.

And the sound, obviously the the 6K4P at higher B+ compress a bit different than 6BC8. To me, this sounds more like the big brother and its variants, more controlled somehow.

I think next step is rolling out some poor man "T-BAR" adapters for the 6K4P ! that would be nice...!

 

[radiance]

I think next step is rolling out some poor man "T-BAR" adapters for the 6K4P ! that would be nice...!

Just a little more cost and effort for completely redesigned pcb's don't you think?

 

[Silvas]

I think next step is rolling out some poor man "T-BAR" adapters for the 6K4P ! that would be nice...!

Just a little more cost and effort for completely redesigned pcb's don't you think?

Possibly.

But, how many PM builders would love something like this without having to start another whole build ? me included, but to be honest, i´m quite happy with the stock unit. You just have to be careful with your levels.

I like the idea of a Poor Man V2. WIth all the improvements suggested here. I Would like one !

 

[Kingston]

I think next step is rolling out some poor man "T-BAR" adapters for the 6K4P ! that would be nice...!

Just a little more cost and effort for completely redesigned pcb's don't you think?

It still needs a more able sidechain, with more voltage swing to use up all the range available with 6K4P. I'm very close to ditching the "poorman legacy" and moving on to a completely new design.

I did say that 6BA6 (and so 6K4P) is not suitable in the PoorMan. The grid voltage swing needed to have -10dB of effective gain reduction is far more than 12Volts.
How did you check the gain reduction ? With the Vumeter or with a real Gain meter ? Did you use the SCAB or stock Sidechain ?

I don't know why you keep saying that. Did you test yourself, or just read some datasheets? I'm using the VU meter to read the gain reduction obviously. It acts the same in compression, or when used with the tube matching rig. 14V = about 17-20dB gain reduction at 160V.

I wonder what's a good way to beef up the sidechain for more voltage swing? Would be interesting to run the tubes closer to the edge.

 

[lolo-m]

I don't know why you keep saying that. Did you test yourself, or just read some datasheets? I'm using the VU meter to read the gain reduction obviously. It acts the same in compression, or when used with the tube matching rig. 14V = about 17-20dB gain reduction at 160V.

I wonder what's a good way to beef up the sidechain for more voltage swing? Would be interesting to run the tubes closer to the edge.

OK, it's time to end some unopened project. I'm working with another member on a prototype based on 6BA6... The voltage swing needs to be a lot more important with this kind of tube than with 6BC8 or ECC189. For example in our prototype, the -20db gain reduction need a CV of -40V and -10db is around -20V, both referenced to the grid voltage for 0db gain reduction.
That would mean in the PM670 a CV swing from -5,5V to -45,5V to get -20db of GR...
The vu resistor have to be choosen well to track correctly the gain reduction. The movement is showing you some compression but maybe not the real amount of compression.
As my prototype use different resistors values and different transfos, I will try tomorrow in the PM setting but I'm quite sure you can't get -22dB without a huge voltage swing... But if it is, that's a really great news  ;D !

 

[[silent:arts]]

guys, may I ask for a separated thread for this discussion?
for sure very interesting, but this is the Poor Man 660 support thread, for the original version ...
it gets a little confusing

 

[emrr]

Here here!  Be cleaner to take it to the 'modify the PM' thread, or start a "PM660 mkII" thread.  A newbie trying to figure out their stock build will have a tough time weeding through all the mod talk in this thread. 

 

[[silent:arts]]

yes, your thread would be the better one for this

 

[Silvas]

The vu resistor have to be choosen well to track correctly the gain reduction. The movement is showing you some compression but maybe not the real amount of compression.

Hey Lolo, can you elaborate a bit more on this ? as i mentioned before, my meter is showing approx 1/2 of the real and measured gain reduction. Of course i would love to have a bit more accurate tracking, but i hadn´t played with the 22K VU resistor yet and my paralell trimmer is 5K. At first i wanted to take the 22K completely out and see...but i´m sure there´s a better way to select the resistor, then zeroing it with the trimmer.

 

[Kingston]

I continue in the other thread. sorry guys. I get a little excited with all these great mods.

 

[[silent:arts]]

http://www.groupdiy.com/index.php?topic=29056.msg351648#msg351648 ;D
thanks

 

[lolo-m]

guys, may I ask for a separated thread for this discussion?
for sure very interesting, but this is the Poor Man 660 support thread, for the original version ...
it gets a little confusing

You're perfectly right. I tried several times to switch this kind of thoughts to the Mod thread but... Sorry  :-[.

 

[[silent:arts]]

I don't want to have you out of this discussion, but with 112 pages it gets difficult for new builders, and for pimping people too  
but I will follow your mods, very interesting experience and discussions.

 

[lolo-m]

The vu resistor have to be choosen well to track correctly the gain reduction. The movement is showing you some compression but maybe not the real amount of compression.

Hey Lolo, can you elaborate a bit more on this ? as i mentioned before, my meter is showing approx 1/2 of the real and measured gain reduction. Of course i would love to have a bit more accurate tracking, but i hadn´t played with the 22K VU resistor yet and my paralell trimmer is 5K. At first i wanted to take the 22K completely out and see...but i´m sure there´s a better way to select the resistor, then zeroing it with the trimmer.

As the thread is a bit long ;D, I re-found that. I hope it can help you:

You've got to understand how's the VUmeter is tracking gain reduction (maybe you already understood that but some of us can be interrested) :
A vu meter is a AC voltmeter (with the rectifier build in). Through R6, the current pumped by the 6BC8 creates a voltage you measure. As the tubes are pumping less current when compressing the voltage drops and the needle goes left. Some little maths : If we call Ib the DC current pumped by the tubes and Ia the current pumped by the tubes to create audio signals voltage on R6 can be calculated with this : Ur6 = [Ib + (Ia - Ia)] x R6.

You can use nearly any Vu or voltmeter here to track compression. On mine, I left R6 on the board and connected on parallel a 500 ohm pot (with 2 points wired together to react as a single resistor). One point of the pot is connected directly to the vumeter, the other point feed the Vu through a 3K6 or 3k9 ohm resistor. It works ! And my Vumeter is an old PEKLY (french brand unfortunately disapeared a long time ago)...

Try this if you didn't and tell us what's happening  .

PS: the needle reacts weirdly only when the 6BC8 are overdriving audio (I mean with no compression).

This setting works or not depending of your 6BC8 curves...