DIY Audio Projects Forum

Valve Pre-Amp Question
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Author:  Jaco [ 27 Jun 2018, 03:59 ]
Post subject:  Valve Pre-Amp Question

Hi all. I have a question. I have a old Marantz SM500DC amplifier , but not the pre-amp anymore.

The amplifier only has inputs and power and speaker select buttons, no volume controls.
The amp works but i will need to make a Pre-amp for it since its basically at full volume when RCA Cables and input get connected. So i need to use the devices volume to control it.

So i was wondering what type of low voltage (like say between 12v and 30v) stereo Valve pre-amp could work? (either one or 2 valves for the respective channels)
Something like the 12au7 Headphone amp maybe? I Like the idea of a valve pre-amp since i still have 2 x 12au7 tubes and a 12ax7 tube .

I need to still check what voltage the amp runs on, maybe i could tap in the for power for the pre-amp.
And ill need to add tone controls as well( bass and Treble).

Any Ideas?

Author:  Soundbrigade [ 27 Jun 2018, 09:39 ]
Post subject:  Re: Valve Pre-Amp Question

6N3 aka ECC85 can often be found i low voltage amps. I think ECC88/6DJ8 is another candidate. Bothe types are made to run on low voltages.

A simple schematic may look like this (plz note that the PSU can be simplified if required):


Also, check if a simple potentiometer in a nice box can do the trick for you.

(*) 6N3 = 6N3P = ECC85 = 6AQ8

Author:  Jaco [ 27 Jun 2018, 15:13 ]
Post subject:  Re: Valve Pre-Amp Question

That looks like it could work.. but i dont have 6n3 tubes..just 12au7 thats why i thought of building another 12au7 based headphone amp and use that.

Never even thought about just using a potentiometer. Maybe worth while testing it.

I still do like the idea of a valve preamp though.

Author:  Jim_O [ 27 Jun 2018, 19:15 ]
Post subject:  Re: Valve Pre-Amp Question

You might want to check out Bruce Heran's Forewatt preamp on this site (tube projects section). The ECC802S tubes that it uses are a variant of the 12AU7. The circuit uses one tube per channel. I built one and the performance is superb. As with most tube gear though, it's not low voltage...the B+ is between 200 and 300 volts.

Author:  steinkare [ 28 Jun 2018, 00:06 ]
Post subject:  Re: Valve Pre-Amp Question

I have built the Forewatt pre with ecc82 tubes. Great DIY tube preamplifier. I have also built "The Bastard" who is based on an article from Danish magazine in the 80-90s. Using around 90 volts and based on the 6J5, 6S2S, L63...There is a guy in Sweden who sells PCB for the preamp. A little modified from the original preamp.Take a look at the internet.

Author:  gofar99 [ 28 Jun 2018, 16:34 ]
Post subject:  Re: Valve Pre-Amp Question

Hi, The ECC802S and ECC82S are similar except the 802 is a long plate design more like vintage 12AU7s and the 82S is a short plate design. I personally find the long plate ones to sound better....but it is just a personal preference. Either could probably be used in a low voltage design, but not as an SRPP. There is just not enough B+ to work with. Without doing the math and making a prototype I would expect the lowest you could go for B+ to be around 125. Some folks run the tubes in a low current range (1ma or so). They do work there, but when I run my distortion analyzer on them it seems that 6-8 ma is best.

I saw one circuit a while back that used the 12AU7 in single ended mode off a voltage quadtrupler from a 12 volt battery and 555 oscillator. Novel, but IMO not the best for hi-fi.

Good listening

Author:  Jaco [ 29 Jun 2018, 01:20 ]
Post subject:  Re: Valve Pre-Amp Question

Hi all thanks for all the advice.

Last night out of curiosity i decided to use my 12au7/irf510 headphone amplifier that a built a while back and just plug it in as the pre-amp..and well it actually sounds good. Works well also. It does bring up the marantz's volume quite high.almost to the level of the original i would guess.However i didnt test volume past say quarter to about half vol on the head amp, since the marantz was then already pretty loud.

The headphone amp is running at 20v at the moment. I also decided that i might go ahead and build a higher voltage preamp since there is quite a lot o space in the amplifier case left for all the extra components.will just need to make a psu for it and tap in from the 240v line of the amp.

Author:  Jaco [ 03 Aug 2018, 02:45 ]
Post subject:  Re: Valve Pre-Amp Question

Hi all.

Since the last time ive posted, i have built the 12au7 headphone amp ( i had the parts extra so that's why i've decided to build it) and have been using it as a pre-amp for the marantz. Still running it at 12v and does work well.

I have recently added a passive baxandall for bass and treble controls which does add to the sound clarity for the valve pre-amp which i like.

However since the passive baxandall has a signal drop of -20dB I then decided to build the active version using a LM358P op-amp which i had. Well it does work and does remove the signal loss.
But i need a bit of help since i'm getting a lot of DC hum using the active baxandall.

AC output (audio signal, in the mV range)side seems fine.
DC is around 5v to 6v on the active baxandall's output which is probably the cause of the DC hum.However on the Valve pre-amps output alone there is less than 10mV DC.
There is also a very high DC output when the pre-amp is switched off, sending the power amp into protect mode.

Im guessing its the "DC thump" of the pre-amp thats now being boosted by the op-amp.

Any suggestions to try and eliminate the High DC on the output and the pre-amp power off problem?

When all is powered up , it does work and sounds very good. I just want to try to eliminate those issues as not to damaged the power amp and or pre-amp.

Author:  Jaco [ 14 Aug 2018, 14:49 ]
Post subject:  Re: Valve Pre-Amp Question

Hi all.. so ive been playing around with the valve pre amp and power amp setup.

I can get the DC output down to about 15mv which does help . And using different power supplies help also but cant seem to get the noise gone.

Theres still a buzz on the output and massive pop when powered off

Any suggestions?

Author:  Jaco [ 22 Jan 2019, 06:21 ]
Post subject:  Re: Valve Pre-Amp Question

Just a little update. Been a while since if built the pre-amp. What i managed to get working was a 19v 3a laptop power supply which is powerfull and small enough to fit. I then made a step down voltage regulator using a adjustable LM 317 [ it was a kit that i had laying around] and used that to bring the voltage down to 12v for the pre-amp and now theres no noise..its dead silent. The pre-amp and amp work very well.. just needs some minor touchups. Play loud enough and has plenty of power. No distortion and bass an treble controls work pretty well to deliver clean crisp sound . Especially.on instruments and vocals . As compared to a marantz 1030 amplifier. So im lretty pleased with it.. it does run a bit hotter so might need a small fan but for now its within thermal limits.

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