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Designing my first tube preamp
Designing my first tube preamp
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Old 5th December 2019, 06:34 AM   #1
Verbstank is offline Verbstank
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Default Designing my first tube preamp

Hi guys,


I posted recently wanting to rework a Stromberg Carlson amp into a bass amp. However, I quickly realized I was biting off way more than I could chew and have rethought my strategy and have decided on building a bass preamp instead, start small and work my way up!


I have some questions I need answered, attached is a schematic for the build.


1. I think I found a good transformer for the project, it is marketed for standalone reverb units. It has 250vac/25ma and 6.3vac/1A heater. I think that that gives me about 250*1.4 = 350v dc after rectifier, which I probably want to step down to around 300v for the 12ax7s. How do I figure out the size of the resistor I should use to drop the voltage before I build the thing or is that more something you meter as you go and find values that way?


1b. Kind of same as first question, with biasing (cathode biasing in particular) is that something that you do after everything is up and running and you can measure the grid/cathode or is there a way to know that value beforehand? It's frustrating that Ohm's law is so simple mathematically and yet I still don't really understand when to use it.



2. Some things I read say that I should have a center tap on the heater winding to be grounded, but that I could create a false center tap by wiring both legs with identical resistors to ground. Is this legit and again how would I determine the value of the resistor?


3. What size filter caps do you recommend for a project like this? I have a 47uf and a 22uf in the schematic, but this is just from borrowing from other peoples designs and I don't have a firm grasp as to why those values or why use 1 vs. 2 vs. 3 filter caps.



4. Please if you would, look over the schematic attached and ridicule me for any omissions or errors. It is still a work in progress. Also, please let me know and recommendations you have for improving the circuit to work better with bass guitar.


Thanks everyone!
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Old 5th December 2019, 08:13 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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1/ The resistor you are looking as is to stop surges mainly aimed at protecting the diodes. Any amount of resistance in the HT line, when the valves are cold, will make little difference. If you are that worried, use a potential divider as regulation is not an issue.
Use a 4k7 as the surge limiter and place a 10k between the two filter capacitors then place a 120k 3W across the second filter capacitor.
2/ the common value used is 120R but anything will do. This stops the heaters from floating to an unwanted voltage.
3/ 1k3 is not a standard value. Most manufacturers using a 100k anode load choose a 2k2 or 1k8 cathode resistor. That brings the anode voltage down to around 80 - 120volts depending on the gain and characteristics of the chosen valve.
I recommend not using JJ's as they are of low quality compared to Sovtek or other non Chinese manufacturers. The 100n coupling capacitors are rather high in value. For bass 47n is enough without causing flutter. the second stage of a 12AX7 has slightly higher gain than the first. If you are using active pickups, don't forget to place a coupling capacitor in series with the first grid. They tend to leak DC voltage and that will give you strange results. Looks like it is from a classic Fender design.
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Old 5th December 2019, 04:26 PM   #3
Verbstank is offline Verbstank
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Quote:
Originally Posted by JonSnell Electronic View Post
1/ The resistor you are looking as is to stop surges mainly aimed at protecting the diodes. Any amount of resistance in the HT line, when the valves are cold, will make little difference. If you are that worried, use a potential divider as regulation is not an issue.
Use a 4k7 as the surge limiter and place a 10k between the two filter capacitors then place a 120k 3W across the second filter capacitor.
2/ the common value used is 120R but anything will do. This stops the heaters from floating to an unwanted voltage.
3/ 1k3 is not a standard value. Most manufacturers using a 100k anode load choose a 2k2 or 1k8 cathode resistor. That brings the anode voltage down to around 80 - 120volts depending on the gain and characteristics of the chosen valve.
I recommend not using JJ's as they are of low quality compared to Sovtek or other non Chinese manufacturers. The 100n coupling capacitors are rather high in value. For bass 47n is enough without causing flutter. the second stage of a 12AX7 has slightly higher gain than the first. If you are using active pickups, don't forget to place a coupling capacitor in series with the first grid. They tend to leak DC voltage and that will give you strange results. Looks like it is from a classic Fender design.

Should I not be worried? I was thinking that 350v would be pushing it for the 12ax7 plates. Or I guess that's what you mean about placing the 120k across the 2nd filter cap, is there a way to know exactly how much voltage that will drop? Sorry does across mean parallel with the filter cap and going to ground?



One of my basses does have active pickups, what size cap would you suggest between the input and grid?


I wont be using crap tubes, I have some Mesa Boogie branded 12ax7s from an old project that I will use as I've always had great experiences with their gear.


Thanks for the comments, I appreciate it.
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Old 5th December 2019, 05:02 PM   #4
petertub is offline petertub  Sweden
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Designing my first tube preamp
Verbstank:


You are on the right track.
A few comments :
- JJ ECC83S will fit perfectly here , don't use substandard brands
- The grid resistor 1M on the first stage , in general i would use somewhat smaller,

250k or so.

- the other stages, don't rely that a potentimeter will be all you need as grid resistor,

connect a 1Meg ( or 500k ) after the viper , this will save the day if the pot fails.
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Old 5th December 2019, 07:37 PM   #5
ddogtheroadkill is offline ddogtheroadkill  Finland
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For power supply design, there is a software tool to help with it, made by fellow diyaudio user, duncanamps. PSUD2 this great piece of software is free for hobbyists and really worth trying.
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Old 5th December 2019, 08:23 PM   #6
Verbstank is offline Verbstank
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Quote:
Originally Posted by ddogtheroadkill View Post
For power supply design, there is a software tool to help with it, made by fellow diyaudio user, duncanamps. PSUD2 this great piece of software is free for hobbyists and really worth trying.
Ok I will try that out when I get home, thanks.

Another question I should have considered a long time ago, is there a big difference between designing a tube pre for use with a solid state power section vs a tube power section? I would like this pre to be able to do both. The only things I can think of are the voltage output of the preamp and the impedance of the ss power amp input. Are these things to stress about or are they usually not a problem? Having 3 gain stages per channel I'd like to think that I could supply ample voltage to any amp, but unsure about the effect of impedance of pre and power, any comments appreciated, will start another Google rampage on this topic when I get home.
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Old 5th December 2019, 08:31 PM   #7
petertub is offline petertub  Sweden
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Designing my first tube preamp
Quote:
Originally Posted by Verbstank View Post
Ok I will try that out when I get home, thanks.

Another question I should have considered a long time ago, is there a big difference between designing a tube pre for use with a solid state power section vs a tube power section? I would like this pre to be able to do both. The only things I can think of are the voltage output of the preamp and the impedance of the ss power amp input. Are these things to stress about or are they usually not a problem? Having 3 gain stages per channel I'd like to think that I could supply ample voltage to any amp, but unsure about the effect of impedance of pre and power, any comments appreciated, will start another Google rampage on this topic when I get home.
Most "classical" tube power amps has a fairly high input impedance, in order
of > 250kohm. Many transistor amps has around 10kohm, this is a nostopper
for your above preamp. The usual cure is to add a cathode follower.
A single ECC82 and some components will do for both channels.
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Old 5th December 2019, 11:50 PM   #8
Verbstank is offline Verbstank
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Quote:
Originally Posted by petertub View Post
Most "classical" tube power amps has a fairly high input impedance, in order
of > 250kohm. Many transistor amps has around 10kohm, this is a nostopper
for your above preamp. The usual cure is to add a cathode follower.
A single ECC82 and some components will do for both channels.

Petertub:


Ok so I read through the Valve Wizard's pages on DC-coupled and AC-coupled cathode followers. I amended my schematic and have reattached it, can you tell me if I'm on the right track with this? I still don't really understand how it drops the impedance, but that will come with more reading I guess. Right now I'm just treating it as a reversed normal triode stage, instead of taking the output from the anode and applying the load resistor to the anode, I am taking output from cathode and applying load resistor to cathode. Is this in a nutshell the idea here? I also replaced the last ECC83/12ax7 with an ECC82/12au7 for use as the cathode follower and will use one triode of it for each channel.


Does this now mean that I cannot run this preamp with a tube power amp? Sorry for all the questions!
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Old 6th December 2019, 12:05 AM   #9
rayma is offline rayma  United States
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That circuit won't work, the bass LP filtering should be before the last tube.
The output is then at the top of the 100k, which goes directly to ground.

With a 10k input ss amp, the bass will be reduced in range compared to a tube amp.
Why have the tone controls for the bass output?

Last edited by rayma; 6th December 2019 at 12:10 AM.
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Old 6th December 2019, 12:25 AM   #10
Verbstank is offline Verbstank
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Quote:
Originally Posted by rayma View Post
That circuit won't work, the bass LP filtering should be before the last tube.
The output is then at the top of the 100k, which goes directly to ground.

With a 10k input ss amp, the bass will be reduced in range compared to a tube amp.
Why have the tone controls for the bass output?

So change the .1uf cap to before the 12au7 tube rather than right before the output? Or are you saying change the placement of the tone circuit, possibly put it instead between the two 12ax7 stages? Or am I wildly off-course?
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