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Hi all, wrapping up a build with a PUTB and having some voltage/volume issues.
First off, when I engage the circuit the volume drops a significant amount.
Additionally the closest I can get the bias to the recommended -7v is -5.46 with the bias pot turned all the way counterclockwise gets quieter/less fuzzy as i turn it up. I hope it’s not a transistor issue as i”ve gone through a couple sets trying to make it work.
Is this typical for the circuit or am I having some issues here?
Transistor specs below:
Q1: MP-426 Hfe-77 lceo-131uA
Q2: MP-426 Hfe-85 lceo-201uA
Q3: MP-426 Hfe-181 lceo-144uAIs this typical for the circuit or am I having some issues here?
No it’s not typical. My simple rule is that I do not release anything that might have an issue. I have had that circuit in circulation since 2010 so if it had issues it would be noted in the forum.
I always suggest testing each circuit of a combo build on its own first.
What is your voltage at the 9v pad of the PUTB?
List your voltages at Pin 5 and Pin 8 o f the IC.
I cannot see what exactly is happening here.
Hopefully the power jack lugs are not touching the enclosure.
What transistor type do you have in Q3 it’s not an MP42B
What did you use to determine the transistor pinouts
All your component values and orientation that I can see look correct
I remember this from Reddit a while back I’m mcknib on there
Good point Billy. I missed that.
Two of the transistors are indeed Russian so you need to put the transistor leads in the correct slots.
Look up the datasheet for your Russian transistors and verify the lead and place in the circuit accordingly.
On Russian datasheets:
K = Collector,
= EmitterHad some time to get back to this one today and I’ll try to answer everything here:
Voltage on pin 5 = -8.95
voltage on pin 8 = 8.99As I forgot to compensate the the center post on the 1590DD enclosure my DC jack is going through the post. When I took the picture it wasn’t glued in so it was floating around. At this time the posts are not contacting the enclosure.
I purchased my transistors in a set from an Ebay seller and verified Q1 and Q2, the MP42bs, pinouts a few different places (small bear for example) and Q3 has the markings “GI8G 6438”. I cannot find any other transistor with those exact markings but I believe the tab on Q3 is indicating the emitter and it’s possibly a 2N6438
I did a deep dive onto many forums and saw a handful of posts about bender builds that seemed quiet and an issue that came up was too much leakage in Q2, with that in mind i ordered a handful of other transistors to hopefully try some mixing and matching to see if the issue lies in the transistor set.
New transistors arrived today and I dropped them in to a pleasant surprise. I’ve got the voltage sitting around -8.5v with no volume loss. Sounds absolutely wild.
Q1 – western Electric 12 Series hfe 73, lkg 220ua
Q2 – Western Electric 12 Series hfe 94, lkg 128ua
Q3 – Same as above, GI8G 6438, hfe 181, lkg 144ua
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