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Old 17th February 2005, 07:17 AM   #1
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Default opamp driver stage

Been thinking lately about the tube amps I have built and have come to the conclusion that the only way to do a tube amp right will require a solid state voltage amp/driver stage driving the tube output....(garbadge in garbadge out) as i cannot stand a noisy amp (hum etc...)

Any suggestions and or schematics on how to do this?


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Old 17th February 2005, 11:00 AM   #2
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Whole things should be quite sophisticated Are u sure???
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Old 17th February 2005, 02:28 PM   #3
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garbadge in garbadge out
A solid state voltage amp does not bode well with your statement above. You'll also find very little enthusiasm for such a project among tube diyers.
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Old 17th February 2005, 03:58 PM   #4
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John, not to be rude but.....whats the point of this fourm????

I think there might be merrit to what im trying to do as the voltage gain/driver stage is everhting in an amp IMMHO. I can see increased dynamics less noise, wider bandwith etc.....

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Old 17th February 2005, 04:38 PM   #5
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I can see increased dynamics less noise, wider bandwith etc.....
Increased dynamics?! What are you talking about? A simple common catode tube stage can swing a hundred volt easily. Use a choke/transformer load and this doubles. Low distortion, great bandwith and all this without feedback. Solid state can do that?

Noise? Why is noise a consideration in a high level stage? People build microvolt headamps with tubes...

If you use a SS front end which will probably determine the prevailing sonic character, what is the point of putting up with tube output and transformers?
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Old 17th February 2005, 06:17 PM   #6
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Please don't be so negative. You might not like the idea, and that's fine, but that is only your opinion.

If you don't like it, just don't post in this thread. We like to encourage and guide new projects, who knows what might come of them?
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
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Old 17th February 2005, 09:24 PM   #7
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IMHO, ultimately transistor voltage gain stages have lower noise than tube, but design, component selection and construction influence this more than the device technology, and for a power amp both should offer adequate noise performance.

For any device technology, resistive loaded single ended gain stages do not reject power supply noise so a high degree of power filtering is needed for low hum, current source loaded gain stages can have high power supply noise rejection but the current source itself can add excess noise, Gary Pimm is an expert on current source loads. Differential gain stages have good power supply noise rejection if a current source or high value resistance is used between the common cathodes and the negative power supply rail.

Filament supplies for input tubes should be well filtered and preferably regulated DC for low noise and hum, a floating filament supply referenced to about 50VDC above the cathode helps in hum rejection. For lowest input stage hum a separate filament supply transformer may be needed to avoid circulating capacitive currents between transformer windings especially with capacitive input power supply filters.

Ground management is of utmost importance for low hum, you cannot rely on tying all ground points to a steel chassis,Kuei Yang Wang's post is a brief introduction to the correct approach.

There are some examples of solid state drivers for tube outputs, the Sutherland driver board for Dynaco ST-70 comes to mind, I recall a description of its design using JFETs cascoded with MOSFETs, however I don't have a schematic and have not seen one on the web. SY has posted a hybrid driver design here, search for SYclotron. JFETs cascoded with tubes are commonly used for low level preamps and seem well accepted by tubeophiles, Allen Wright for example.

Since you already have some tube amp projects perhaps you can start by examining your ground management and power supply designs for possible improvement. If you want to start over, I suggest you try whatever approach suits your fancy, you will learn more if you try whatever you have time and patience for.
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Old 18th February 2005, 07:01 AM   #8
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Hi Acoustat,

Don't let anyone put you off.
I built a fantastic 4x KT88 AB1 40% Ultralinear this way some 30 years ago. It had part self + part fixed bias.
It is still in my roofspace ... ! somewhere !

Each 2x KT88 output half was driven by 1/2 a Mullard ECC82 (12AU7) cathode follower, which was directly coupled to and bootstrapped its first 1/2 operating as conventional common cathode amplifier. The bootstrap completed the 500uF+1/4A choke+500uF smoothed power rail rejection at the driving amplifier. Yes I was operating beyond manufacturers ratings, but genuine Mullard's were okay.

I measured to the ECC82 cathode outputs without the output valves, and without any NFB found less than 0.1% distortion for 100Vrms drive, which was far more than the KT88s would ever need. The amplfier was humless (no 6.3Vac at low level circuitry), and almost hissless without signal input.

Each ECC82 input grid was fed by the collectors of a simple single differential stage, and there was enough spare gain for NFB. The transistors were powered by the output stage fixed grid bias supply.

Sure we still use differential input stages in high power SS amplifier designs, and it is not this stage that imparts characteristics. Besides, the input transistor stage was so much faster than the following valve stages or any alternative input tube type that they did not - could not - impart any sonic character.

This was my reference amplifier for many years, and I felt particulary decieved by Quad when their supposedly 'ideal' (at the time) 405 amplifier was so inferior beside it. The Quad was loud enough alright, and okay for pop parties, but for serious listening - yeough.

I did try driving output valve grids with BF259s but they were not as clean with the grid loading.

Cheers .......... Graham.
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Old 18th February 2005, 07:29 AM   #9
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Default SS vs tube

Sometimes I was talking with chiefdesigner from KR Enterprise and he said to me, that in their amps is all ( except output tube ) SS, 'cos it give better sonic results. " Let's think make it better " ( Philips ) and don't stay in " old fashion " .
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Old 22nd February 2005, 03:01 AM   #10
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Default Notes and Questions

There was a good SS front end, Ultralinear connected EL34 Output design in Electronics World about 12 months ago. Used Lundahl Output Transformers.

Can you describe the bootstrap connection from the direct coupled cathode follower back to the common cathode previous stage in your KT88 Amp - there are many ways to skin the cat..

Did you split the load resistor of the common cathode stage and capacitively couple the cathode follower output to the junction of the 2 resistors?
Did you sit the cathode follower on top of the common cathode stage ie a Mu Stage connection?
Did you AC couple the cathode follower output back to the cathode of the Common Cathode Stage?
Something else entirely?

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