Sekova FA-II/Crestwood Fuzz/Apollo Fuzz question

[gigimarga]

Hello,

I finished a Sekova FA-II using this schematic (thx to analogguru):  http://analogguru.an.ohost.de/001/schematics/Sekova_FA-II.gif.

I used two SFT242 with gain between 70-90 and it sounds pretty OK. I have some doubts about it because it sounds a little gated on some settings

My question is if somebody has a soudclip of it (I found one made by Dragonfly on DAM forum, but the link is broken) or voltages of the original?

Thx a lot all!

[Mark Hammer]

Looking at the schematic, the junction of R10 and R5 seems a little odd to me.  If the path between R10 and ground is only 100R, why even bother having R10?  Something just doesn't seem right, although if anyone can offer an explanation of why it IS right, I'd be appreciative.

[gigimarga]

Thx Mark for your kindness again!

Here you can find some photos and some informations: http://www.stompboxes.co.uk/forum/viewtopic.php?f=23&t=525.
Almost the same informations you can find on "another forum" if you search for "Crestwood".

The fuzz has a low output volume, so it's possible that this resistor to be the cause? Maybe there is no connection between R4/R5 and C3?

[Gus]

R5 and R10 are part of the feedback around the cascaded gain stages

Again when having a "problem" post transistor voltages and sometime other node voltages

Q1 and Q2 collector, emitter and base voltages ref to ground, the supply voltage and voltage at the R5 and R4 node.

[Mark Hammer]

The fuzz has a low output volume, so it's possible that this resistor to be the cause? Maybe there is no connection between R4/R5 and C3?

Look closely, and you will see that there is a GE diode pair after Q2, so that will limit output level.  Then, it is followed by a midscoop filter formed by R11/R12/C8/C9, which will result in considerable signal loss.  The absence of any gain-recovery stage after the tone circuit means that there will be only very modest output level from the circuit.

Just like the Shin-Ei FY-2, if you insert a variable resistance between C9 and ground, you will decrease the depth of the midscoop and, as a result, increase the output volume a bit.  Of course, because the GE diode pair places a +/-220mv (or close to that) ceiling on maximum signal, there is only so much volume increase that this inserted resistance will be able to accomplish.   A variable resistance of 5k between C9 and ground will be enough to do what you need to do.

[gigimarga]

Thx a lot both of you, Mark and Gus, for your help.
I thought that diodes and midscoop network could be the cause of the low volume, but I wasn't sure.

Anyway, analogguru said in a post that some versions have 33K instead of 27K resistors, 4.7K instead of 5.6K resistors and Si diodes instead of the Ge ones.

My voltages, using a 8.70 battery, are:

Q1: C = -5.75     B = -3.08    E = -2.95
Q2: C = -5.70     B = -3.03    E = -2.93

R4/R5 node: -0.06

Thx again!

[analogguru]

Looking at the schematic, the junction of R10 and R5 seems a little odd to me.  If the path between R10 and ground is only 100R, why even bother having R10?  Something just doesn't seem right, although if anyone can offer an explanation of why it IS right, I'd be appreciative.

R10/R5 form an AC-feedback with the purpose of limiting the overall gain of the two cascaded gain-stages to 220 (~ 46 dB).
For AC-signals R4 is shunted with the bypass cap C3.

analogguru

[gigimarga]

Thx analogguru!

[gigimarga]

Nodoby have saved Dragonfly's clip?

[gigimarga]

 

[gigimarga]

The voltages seems OK?

[analogguru]

@gigimarga
I have seen your pm but after this:
http://www.diystompboxes.com/smfforum/index.php?topic=77626.40
I got the impression, that this forum has been degenerated to a kindergarden.

Therefore I decided to stop visiting this kindergarden and answering questions here for a longer period of time.
Feel free to contact me in the appropriate thread at the webadress which can be found in the schematic - as long as my schematics don´t get censored too over here - and I will try to help you.

goodbye,
analogguru

[gigimarga]

Thx a lot analogguru!