Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fiftynine on August 22, 2010, 06:18:34 am
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Can I take the normal channels output to the PI (where the Dan Torres mod starts) and send it to the input of the bright channel (pin 2, V2) for extra gain? So plugged into the normal channel, I'll have twice the gain (plus reverb, bright cap bypassed) and plugged into the bright channel, there will be no change.
Does that sound right? Are there any issues with this I've not spotted? Has anyone tried it?
Thanks.
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When you start getting multiple cascading gain stages, you risk oscillation problems signficantly.
And if you will note with Dumble or Bad Cat amps, when they have 4 plus gain stages they tend to do three things:
http://www.schematicheaven.com/newamps/badcat_hotcat_30.pdf
- they have multiple volume controls OR dropping resistors
- they have smaller size cathode caps not the standard 22uf more like 1uf to 4.7uf.
- they will sometimes use snubbing resistors to help prevent oscillations
And the layout becomes more critical to prevent oscillation problems.
Multiple gain stages not done correctly can make an amp sound like a very unmusical chain saw.
With respect, Tubenit
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When will I learn....trying to second guess Leo Fender :rolleyes:
Cheers, tubenit.
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59 - It has been done. You could use the Normal channel Bright switch to connect/disconnect the two channels. Do a search over on AMPAGE. I'm pretty sure I saw a discussion there within the past year.
Nevertheless, ALL of sluckey's tubenits points are solid.
Chip
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"ALL of sluckey's points are solid"
And tubenits were too, right? :huh: :wink:
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"ALL of sluckey's points are solid"
And tubenits were too, right? :huh: :wink:
OK, so my brain was elsewhere... Sorry about that! :embarrassed:
Chip
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Chip - no offense meant, I was searching all over for sluckeys comments.. LOL
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No offense taken!
I wouldn't think of doing this to a vintage Deluxe Reverb, but here's an untested idea to get more gain from the Normal channel. MOSFET cathode follower (Google "MOSFET Follies") reduces loss from tone stack. Normal channel tied in right after the reverb mixer (3.3meg resistor) so it gets boost from same # of gain stages. The 1.5meg resistor shown forms a voltage divider with the 220K going to ground at the end of the reverb circuit, so the 1.5meg would need to be adjusted to get proper input levels for the next stage. My guess is 2.2meg or higher might be "better" in the sense of not hitting that stage too hard.
Oh, if you built it this way, the Normal channel would have tremolo but not reverb.
Just for fun...
Chip
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Blimey. You lot should get together and design something contemporary.
Are we saying a preamp is not just a preamp and the design needs to be matched to the following stages? i.e. if you made a pedal that took the normal channel Blackface preamp and plugged into the bright channel, it wouldn't just 'work'?
It's not a vintage DR by the way, just a recent AB763 build but I was trying to avoid major changes like a PPIMV for example.
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Blimey. You lot should get together and design something contemporary.
Are we saying a preamp is not just a preamp and the design needs to be matched to the following stages? i.e. if you made a pedal that took the normal channel Blackface preamp and plugged into the bright channel, it wouldn't just 'work'?
It's not a vintage DR by the way, just a recent AB763 build but I was trying to avoid major changes like a PPIMV for example.
Edited by Chip after realizing I had "heard" sarcasm where none was intended. I apologize for my mistake.
Huh? No one said "you shouldn't try this" or "we aren't interested in thinking outside the box here." Please consider the following comments as constructive criticism.
In the first place, the concept I threw out is a significant departure from an AB763 and the cumulative gain hitting the phase inverter would be a LOT higher than the Vibrato channel (let alone the Normal channel). No, I haven't built it but it might be fun to try. You may be able to implement it in an existing AB763 layout without too much ugliness. Depends on your specific layout.
Secondly, I've been coming to grips with the fact that the entire circuit really should be designed as a whole. In fact, it appears to be better to start at the power amp and work "backwards" through the circuit. You need to know what signal level the power tube(s) require before you can design the PI/driver stage. Then you'll know what signal level range is acceptable for the PI/driver and so on, working your way back to the input jack. So "No", "a preamp is not just a preamp". For the change I threw together, you would need to experiment with the 1.5meg resistor to get a signal level which works for you in your amp.
Third, the initial question wasn't "can we design a preamp with 5 gain stages and reverb?" It was "can I do X to a Deluxe Reverb?" If you are starting with Fender's AB763 layout, cascading 5 gain stages may be problematic due to layout problems. The layout simply wasn't designed for a high gain amp. Personally, I've had interference and cross-talk problems with several Blackface Fender builds using Hoffman's layout as a starting point, and that's "better" than the vintage Fender layout IMHO. The reverb recovery stage is especially sensitive and suceptible to squealing, etc. And the change you proposed could be more complicated than adding a PPIMV depending upon your existing layout.
Fourth, have you paid any attention to the Little Wing, HoSo56, or Tweed Overdrive Special circuits which guys here have worked out? Those are really cool, contemporary designs from my perspective.
AX84.com offers several preamps and several poweramps which are designed to be (more or less) mix & match. There are also several complete amp designs like the SEL which has 4 cascaded gain stages IIRC. Plenty of gain there. They have layouts which are proven to work with the gain levels of the respective circuits. There's some really neat design work going on over there as well.
Everybody here asks questions - that's what the forum is here for. However, asking a question and then getting a bit testy when you don't like the responses can be annoying. Especially when I took the time to think up an alternative which might work OK and draw up the proposed modification.
My response is intended to be helpful, not just critical. I hope my attempt is successful.
Chip
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Chip, I really wasn't being sarcastic. Honestly. You and tubenit made some neat and very educated suggestions (as you have done for me in the past) without even having to think about it. My 'contemporary' comment was a compliment suggesting you should all put your ideas together and create something with a twist, non-profit or otherwise.
Reading my first sentence again, its easy to see how it came over the wrong way. I'm that shocked I upset you I haven't even read the rest of your reply yet. I'll do it now.
Apologies.
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Chip, PM sent.
"designed as a whole". Hammer, nail, head.
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The new Super-Sonic 22 channel 2 (based on the Prosonics originally I think) has cascading stages and reverb and can sound close to channel 1 which is supposed to sound like a DR (but it doesn't quite). Here's the lengths Fender had to go to, to achieve that.
(http://i795.photobucket.com/albums/yy238/little_schmuck/ss03.jpg)
(http://i795.photobucket.com/albums/yy238/little_schmuck/ss04.jpg)
(http://i795.photobucket.com/albums/yy238/little_schmuck/ss05.jpg)
(http://i795.photobucket.com/albums/yy238/little_schmuck/ss06.jpg)