• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

DIY Tube Preamp question

#84
FWIW, all my early B15N schematics (a B15NA and 3 versions of B15NC) show a 4M treble pot on channel 1 and a 1M treble pot on channel 2. My earliest B15NF schematic is dated 9/65 and also shows the 4M and 1M pots. My other B15NF schematic dated 1/66 shows both treble pots as 1M. Since the preamp circuit changed between the 'NC and 'NF versions I'm wondering if the 4M pot in my early B15NF schematic should have been changed but was missed at the time?
 
#86
Vintage-Blue said:
FWIW, all my early B15N schematics (a B15NA and 3 versions of B15NC) show a 4M treble pot on channel 1 and a 1M treble pot on channel 2. My earliest B15NF schematic is dated 9/65 and also shows the 4M and 1M pots. My other B15NF schematic dated 1/66 shows both treble pots as 1M. Since the preamp circuit changed between the 'NC and 'NF versions I'm wondering if the 4M pot in my early B15NF schematic should have been changed but was missed at the time?
Click to expand...

1962 B15-N has 4M in both channels:

Invalid Link Removed

1964 B15-NC has 4M in both channels:

Invalid Link Removed

1968 B15-N has 1M both channels:

Invalid Link Removed

1968 B15-NF has 1M both channels:

Invalid Link Removed

1971 B15-S has 1M both channels:

Invalid Link Removed




The topology is different between those with 4M and 1M treble pots -

those with 4M are as follows:

1/2 6sl7 -> tone stack ->1/2 6sl7 -> volume ->phase splitter

Those with 1M are as follows:

1/2 6sl7 -> volume -> 1/2 6sl7 -> tone stack -> phase splitter
 
#87
I believe those early Piazza schematics are incorrect, at least relative to the original schematics:

b15na.jpg


b15nc_1.jpg


b15nc_2.jpg


b15nc_3.jpg


I wonder if Joe was working from the original Ampeg schematics or if he redrew the schematics from looking at the actual amplifiers? It could be that the actual components didn't match what was on the Ampeg schematic.
 
#89
Vintage-Blue said:
I believe those early Piazza schematics are incorrect, at least relative to the original schematics:
Click to expand...

Looks like it.



Vintage-Blue said:
I wonder if Joe was working from the original Ampeg schematics or if he redrew the schematics from looking at the actual amplifiers? It could be that the actual components didn't match what was on the Ampeg schematic.
Click to expand...

Possible... or the old devil of drawing one channel first, copy/pasting it in CAD, then forgetting to modify one of the values...
 
#91
Will changing my 1m treble pot to a 4m pot do much difference? I cant hear any change of sound when I adjust the treble pot so I have a feeling that Ive got something wrong with my soldering. The bass pot works great though.

Another question, that .02uf cap that goes to the tone circuit, do I need that? I have it wired in at the moment. What does that capacitor do?
 
#94
Bass4lyf said:
Will changing my 1m treble pot to a 4m pot do much difference? I cant hear any change of sound when I adjust the treble pot so I have a feeling that Ive got something wrong with my soldering. The bass pot works great though.

Another question, that .02uf cap that goes to the tone circuit, do I need that? I have it wired in at the moment. What does that capacitor do?
Click to expand...

I haven't been following this thread all the way through so forgive me if this has been covered already. Double check the capacitor values for the treble pot. For the earlier circuits with tht 4m pots, the caps are 1/10 the value of the later circuits with the 1m pots. If you are using the 47pf and 470pf caps with the 1m pots, the treble turnover frequency will be way too high and you won't hear much difference.

EDIT: Try to double check against the original Ampeg schematics. I've found these capacitor values to be wrong in some of the re-draws.
 
#96
gerryjazzman said:
If you are using the 47pf and 470pf caps with the 1m pots, the treble turnover frequency will be way too high and you won't hear much difference.
.
Click to expand...

Thats exactly what I am using. So if I change the pot to a 4m, I will beable to hear a decent change in treble when I adjust the pot?

If I were to keep the 1m pot, what caps would I put in place of the 47pf and 470pf?
 
#97
Bass4lyf said:
Thats exactly what I am using. So if I change the pot to a 4m, I will beable to hear a decent change in treble when I adjust the pot?

If I were to keep the 1m pot, what caps would I put in place of the 47pf and 470pf?
Click to expand...

The 4M pot would probably work then. If you keep the 1M pot, the caps should be 470pf and 4700pf respectively (10x higher values). That would be my recommendation since that is what Ampeg went to for their later amps. Even carries through the V series (v4, V4B) and the SVT.
 
#100
Hey guys, its been a while since my last post. The preamp has been built and everything is working nicely. When my internet is faster I will post picks up of the build process.

Anyway, the gain from the output is way too high. I wired a bypass switch into the preamp and have been comparing. Ive noticed that when the preamp is plugged into my bass amp, I have no problems, works well. But when I plug the preamp into my recording interface, even though the interface is not clipping, it distorts alot so I have to use a second pedal to turn it down. What can I do to bring down the gain?
 

Latest posts

Top Bottom
Back