DIY Tube Preamp question

[BassmanPaul]

The same company has a suitable power supply board and that might be a better way to go. The pre-amp tube heaters will be quite happy with 12,6V AC though the residual hum level might be a bit higher. This is a stereo board based on a traditional HiFi tube pre-amp. That means you have two channels where you only need one for a bass. There is also no method for controlling EQ.

Personally I think the first board would have been more suitable. You could have used channel 1 to pre-amplify the bass and feed a set of tone controls and channel 2 to regain the signal lost in the EQ and buffer the output jack.

You can use the board you bought the same way but I think you may well end up with too much total gain. The input impedance is too low at less than 100KΩ but you can try it as is and see how it works.

The output impedance should be able to drive a SS power amp. If not you should be able to add a step down audio transformer to drop it to an acceptable level.

Paul

 

[Bass4lyf]

The input impedance is too low at less than 100KΩ

Can I change the input impedance?


And yea, approx 230volts in aus

 

[BassmanPaul]

Remove the pot on channel one and use that for the input. The 470KΩ grid resistor will be a good match for a bass.

Paul

 

[SurferJoe46]

<snip> You don't have to DC rectify the 12.6V, but you can if you want. If you want the simplest solutin, you can feed the 12.6V with AC since it's just the heaters for the tubes. <snip>
Chris

Isn't there going to be a little vestigial AC hum that way, even though the heaters don't really care one way or the other? I always though that any AC inside the tube envelope would carry over.

Some of the old-fashioned radios had AC there and as the paper/wax caps got old, then the AC hum appeared and just got worse. But a lot of the upper-end radios and Hi-Fi s (young ones can ask what that is if they like) used rectified heater supplies.

 

[throbbinnut]

Yeah, DC is better as far as preventing hum. Some of the boutique stuff used/uses DC heater supplies to minimize any AC noise coupling into the tube guts, but you don't have to do it to get satisfactory results. Look at the old Ampeg, Fender, and Marshall amps - all AC heater supplies.

Chris

 

[rickdog]

Anyway, I purchased this already built pcb board and recieved it in the mail yesterday...

It says it needs 280v DC to power the tubes and another 12.6v DC. I just want to make sure Im on the right track with this.

Okay, so last week I wrote that tube circuits are easy... and this week I have to say this one isn't quite so easy

The complicating factor in this one is the DC coupling between the second 12AX7 stage and the 12AU7 sections. The plate of that 12AX7 is going to be somewhere around 140V, it's directly coupled to the grid of the top 12AU7, so the cathode will be a few volts higher. On the other hand, the cathode of the lower 12AU7 will be only a few volts above ground.

This means the two cathodes will be (very roughly) 140-150V apart. The maximum steady heater to cathode voltage for a 12AU7 (according to my old RCA tube handbook) is 100V.

This means that the heater supply has to be biased up to something around 70V (splitting the difference between the two cathodes).

Bottom line: you need at least the schematic for the matching power supply. You might want to just buy that power supply.

Also, as an aside, it looks to me like this preamp will have way more gain than you need! I'd suggest keeping the input potentiometer (replace it with a higher value to avoid loading your pickups too much), and adding another potentiometer to control the signal level to the second 12AX7 stage. Then you can control how much each of those stages is overdriven.

 

[BassmanPaul]

adding another potentiometer to control the signal level to the second 12AX7 stage. Then you can control how much each of those stages is overdriven.

Putting a pot as you suggest will place it within the feedback loop. This, I feel, could cause instability. Better would be to replace the 471K&#937; resistor at the first grid with something like 2M2&#937; and replace the pot with a 1M&#937; audio (log) taper pot. Let's call it gain. Output from that channel can feed a standard three way + volume tone stack feeding channel 2. Output of channel 2 tp output jack If the end result still has too much gain then we can look at cutting that down a bit.

Paul

 

[BassmanPaul]

My main concern apart from the gain issue is the decoupling of the high voltage supply. Having two sets of gain stages sharing the same B+ supply can also lead to instability. Again that too is fixable.

Paul

 

[BassmanPaul]

This means the two cathodes will be (very roughly) 140-150V apart. The maximum steady heater to cathode voltage for a 12AU7 (according to my old RCA tube handbook) is 100V.

This means that the heater supply has to be biased up to something around 70V (splitting the difference between the two cathodes).

Absolutely correct Rick, all "totem pole" topologies should have their heaters biased hot with regard to chassis. I had failed to account for that! ;(

Paul

 

[rickdog]

Putting a pot as you suggest will place it within the feedback loop. This, I feel, could cause instability.

Oops, good catch! I was so focused on the heater-cathode voltage I forgot about the feedback loop.

Good news is that the feedback means the overall gain will be much more reasonable.

 

[Bass4lyf]

This means the two cathodes will be (very roughly) 140-150V apart. The maximum steady heater to cathode voltage for a 12AU7 (according to my old RCA tube handbook) is 100V.

ok, im too tired to think properly right now, could i fix this problem by swapping the 12au7's for 12ax7's? have two 12ax7 per channel? They have the same pins dont they?

EDIT: I got emailed the schematic for the recommended power supply, should be attatched on this post.

Looking at everything now, Im seeing that it would probably be the simplest and easiest way to just by this power supply. I was kinda hoping I didnt have to so I could just whip up some other rectifier for alot cheaper. Does this power supply include the filter caps?

 

[BassmanPaul]

Very much an odd ball design. A bridge rectifier implemented using a tube and a pair of SS rectifiers. The heater supply is grounded. B+ supply is regulated.

Paul

 

[Bass4lyf]

Very much an odd ball design. A bridge rectifier implemented using a tube and a pair of SS rectifiers. The heater supply is grounded. B+ supply is regulated.

Paul

It will work with my preamp though wont it?

 

[BassmanPaul]

Yes but the 12AU7 will have its heater cathode voltage rating exceeded. Most likely you'll never have a problem.

Paul

 

[Bass4lyf]

alright. Also I emailed them asking if they had the correct transformer to go with the power supply and preamp, they replied back with this: http://cgi.ebay.com.au/280-250V-Aud...pt=Vintage_Electronics_R2&hash=item483a5d84fb

The only thing Im worried about is that the transformers input is 220volts whereas me living in Australia the voltage here is around 230/240v. I asked them if that would be a problem, they replied back saying 'it is not a problem with only 10% difference in voltage'. What do you think?

 

[BassmanPaul]

Should be fine. Your ac levels will be running a tad high but as all the supplies are regulated the pre-amp will know no difference. Just make sure that the voltages at each primary filter is not higher than the caps voltage rating.

Paul

 

[Bass4lyf]

alright sweet. Il buy them this arvo. Thankyou soo much Paul and the rest of you guys for your help, means alot! I will be posting pictures of the build process on here so yous can see. Hopefully all goes well

 

[Bass4lyf]

I bought everything, just gotta wait for it to come in the mail.

Now for the equalizer section. What would be an ideal equalizer to use for this preamp? Something that works best with bass guitar. Im thinking either the baxandall or the fender/marshall style circuit. I would prefer a mid control but it really isnt necessary and Id like to beable to boost cut frequencies. The baxandall would be easier and simpler but ive read that it is more of a hifi equalizer and doesnt work with anything else that well. I have got the duncan tone stack calculator and have been fiddling around with that but yea. If I have the right values of resistors/capacitors to suit a bass guitar, could a baxandall eq work really nicely?

 

[BassmanPaul]

Take a look at the Ampeg B15 tone circuit. That's what I used in my own pre-amp. Just use the treble and bass controls. Simple circuit using few components and is nice for bass.

Paul

 

[Bass4lyf]

Better would be to replace the 471K&#937; resistor at the first grid with something like 2M2&#937; and replace the pot with a 1M&#937; audio (log) taper pot. Let's call it gain.
Paul

Can I use any 2M2 resistor for this or does it have to take a certain high voltage?
Ive read that you can use anything but im still unsure about that.

Should I be using log pots for everything? (bass, treble, gain, vol)
I assumed linear would be more accurate but when I tested how linear and log pots work in a tonestack calculator in the baxandall design, when the pots were log, the eq was fairly flat, but when changed to linear pots, there was either a cut or boost (cant remember) in the bass and treble.

Thanks