Multi-purpose tube amp (line and guitar inputs) - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th March 2006, 12:54 AM   #1
diyAudio Member
 
smsmonster's Avatar
 
Join Date: Mar 2006
Location: NL
Default Multi-purpose tube amp (line and guitar inputs)

Hey guys,

As I was thinking of a design for a simple tube amplifier, I thought it might be a good idea to have two inputs on it, one line input (eg. CD-player) and one guitar input (so there has to be a guitar preamp in it). So basically:

(guitar preamp) > preamp > phase shifter > push-pull stage > output transformer

The guitar preamp can be switched on or off accordingly.

I've got a couple of tubes, some E88CC's and RCA6350's, which are both twin triodes. My goal is not to produce some kind of high-end amplifier with components over $200, it's just for fun.

Schematic

What I'd like to know is: is my design going to work? Are there any special points I should take into consideration or other suggestions?

Because no attention is spent on tube design in school, I don't know how to calculate them so my resistor values might be a bit off. And what about the input impedances? Are they in the right direction?

On another note: maybe it would be cool to include a switchable tube overdrive. Any ideas how I can achieve this?

By the way i've grounded the low side of the filaments supply to reduce humming.

Thnx

Schematic
  Reply With Quote
Old 16th March 2006, 07:27 AM   #2
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
Don't do it just yet. It will not work as is and is therefore dangerous.



In my opinion, you should read http://www.tubecad.com/articles_2003..._Amplifier.pdf
and read it until you understand it.

If I sound like a parent, I don't mean to. You seem to have the talent. Push on and learn


BTW, welcome to the forum!
  Reply With Quote
Old 16th March 2006, 10:12 AM   #3
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Your design looks ok-ish in principle, but some points below:

1)Your B+ is way too low, you probably need about 300V or so. Try using a 230V 1:1 transformer or put the 2 115V secondaries in series.

2)Also use at least 100uF or so for the first power supply cap for less ripple.

3)Have another dropping resistor and cap for the preamp stage power supply to isolate it.

4)Also remember plate voltage is V anode - V cathode. Don't be afraid to run the 6350 valves at limits- just be careful of max plate dissipation (Va-Vc) x Ic.

5)You need coupling capacitors and grid leak resistors on the first two valves.

6) Ground one side of the speaker, and run a negative feedback loop from the + speaker terminal to the cathode of V1A, you can switch it off if you want more distortion, on when you want hi-fi.

7) Put a volume control in front of V1B, so you can choose to overdrive the preamp, power amp (or both )


The 6350 appears to be like a 12BH7, so you might easily get a couple of usable watts. Run it in class AB (with low idle current) for the most power. As for tube overdrive, just switch in that extra stage. Put cathode bypass caps on the preamp valves for more gain (say 100uF on V1A and 1uF on V4A ) Check the resistor values, I don't know what they should be sorry but have a look around, you'll get an idea.

Have a look at some Fender schematics for more ideas... You could switch to a 'long-tailed pair' phase inverter for more drive.

Looks like it would work in principle though Good luck
  Reply With Quote
Old 16th March 2006, 07:04 PM   #4
diyAudio Member
 
smsmonster's Avatar
 
Join Date: Mar 2006
Location: NL
Allright, those are some good hints
I've adapted my design accordingly:
>>Check it out here<<


Quote:
Your design looks ok-ish in principle, but some points below:

1)Your B+ is way too low, you probably need about 300V or so. Try using a 230V 1:1 transformer

or put the 2 115V secondaries in series.
Maximum ratings for the 6350:
Plate voltage: 330V
Cathode current: 27.5mA (peak 333mA)
Plate dissipation: 3.85W / triode
I think I might damage it if I increase B+.. See 4)
Quote:
2)Also use at least 100uF or so for the first power supply cap for less ripple.
Right, forgot to change that.. I've switched to a 220uF 250V cap.
Quote:
3)Have another dropping resistor and cap for the preamp stage power supply to isolate it.
Added a 100uF cap and a resistor..
Quote:
4)Also remember plate voltage is V anode - V cathode. Don't be afraid to run the 6350 valves at

limits- just be careful of max plate dissipation (Va-Vc) x Ic.
Max dissipation is 3.85W, so that makes 154V @ 25mA.. Does that sound about right? The datasheet mentions some "Characteristics range values for equipment design", which state that at Va=150V and Vg=-5, Ip is(should be?) between 6 and 16mA.
Quote:
5)You need coupling capacitors and grid leak resistors on the first two valves.
Done
Quote:
6) Ground one side of the speaker, and run a negative feedback loop from the + speaker terminal
to the cathode of V1A, you can switch it off if you want more distortion, on when you want
hi-fi.
I don't really know how.. I'm sorry, I'm rather new to this tube designing.. By the way i've
got half an E88CC left, can I use it for something or should i just run it on one triode?
Quote:
7) Put a volume control in front of V1B, so you can choose to overdrive the preamp, power amp
(or both )
I think I've got it figured out now.. But what makes the difference between overdrive and the "normal situation"? Could I use the second half of V4 to create some kind of switchable clean/overdrive feature?
Quote:

The 6350 appears to be like a 12BH7, so you might easily get a couple of usable watts. Run it

in class AB (with low idle current) for the most power. As for tube overdrive, just switch in

that extra stage. Put cathode bypass caps on the preamp valves for more gain (say 100uF on V1A

and 1uF on V4A ) Check the resistor values, I don't know what they should be sorry but have a

look around, you'll get an idea.

Have a look at some Fender schematics for more ideas... You could switch to a 'long-tailed

pair' phase inverter for more drive.

Looks like it would work in principle though Good luck
I still don't quite understand what determines the class. Okay, of course the idle current (which is when the grid is connected to ground via an 1M resistor and nothing else right?), but how do you determine the idle current? How much is low?

@lndm: Why is it dangerous? is there something wrong with the power supply? By the way that link was a big help, thanks

6350 Datasheet
E88CC Datasheet
  Reply With Quote
Old 16th March 2006, 09:58 PM   #5
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Actually you want to maximise the voltage swing on the output transformer, which means higher voltage at low current. A wild *** guess a B+ of 300 volts should be about right, adjust the cathode resistors to give maybe 5-10mA per section, giving less than 2W disspiation at idle per section. You won't damage them a bit!

Basically these things are like a more powerful 12BH7 which is kind of like 12AU7/ECC82. They can take a beating. The whole class AB thing is harder to explain, but basically if you run them at high voltage and low idle current, you will get more power than if you run them with high idle current in class A. Valves are voltage amplifiers rather than current amplifiers.

As for your input section, you should limit the B+ to about 120V or so, these ECC88s like low voltage apparently. Adjust that dropper resistor R19 to lower the voltage. To use up that extra section, I suggest using a fender type driver design like the 12AT7 at http://www.freeinfosociety.com/elect...enderab673.gif

I don't know much about the ECC88 sorry, so no ideas on what values you would need... But it would have better drive to the output stage with a driver liek that. Here is an interesting schematic http://www.tubes.mynetcologne.de/roe...p_amp_full.gif
from http://www.tubes.mynetcologne.de/roe...ematics_e.html

For negative feedback, add a 100 ohm resistor between the 560 ohm of V1a and ground. Make sure the cap is also raised above ground. Then connect an appropriate resistor from the speaker + terminal to the junction of the 560 and 100ohm resistors. you could start with 10K or so, but for best results calculate how much gain you want. See above schematic.

Its getting there!
  Reply With Quote
Old 16th March 2006, 10:33 PM   #6
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
smsmonster, I was taught in college to have a staunch attitude toward safety. I'd like to move on now (and maybe be helpful?).

The plate resistances for the first three stages get smaller, I see the intention, (and with valves being voltage controlled devices) there is not usually so much variation. I don't use the e88cc, but I assume 15k sounds like a good start. It would be similar for all three stages. You can adjust the cathode resistor until the plate sits at the desired voltage. The link I gave you would set you up IMO, helping you find the ideal Ra and Rk.

You have three stages feeding off the pre supply rail. This can cause regenerative feedback (google motorboating). Try not to run more than two stages off one rail, just repeat R19 and C1.

R19 should be The desired voltage drop divided by the current drawn by the stages it is supplying. Usually between 1k-50k

P1 could probably 250k or 100k. The 1M resistor following may load down the 500k pot giving inaccurate performance. The previous stage will handle it.

Grid stopper resistors. Putting small (typically 100-1000 ohm) resistors at (right at) the grids can eliminate oscillation that occurs due to the input capacitance.

Try not to run the early stages at max diss or max current (for longevity and for sound), it is not necessary.

Also, I would leave the feedback loop until I had the rest working (if you want it at all).

Class A means the amplifying device conducts current throughout the entire wave cycle.

You can measure current by measuring the voltage across a resistor and dividing by the resistance.

Good luck with this.


PS, SHiFTY, just realised I overposted you, got distracted.
  Reply With Quote
Old 17th March 2006, 03:01 AM   #7
diyAudio Member
 
smsmonster's Avatar
 
Join Date: Mar 2006
Location: NL
Hey thanks you guys

I've been calculating and reading all night, I came up with this:
Drawing.

I think I can get it.. But I'd have to know what the gain of V1A has to be, so I have to know what kind of signal I have to provide to the resistor network consisting of R6,7,8,9.
As you can see, I've calculated that the gain of V4A should be 5x to go from 200mV to 1V and that the gain of the phaseshifter (cathodyne) is 0.9X. So the overall gain is 4.5 or 0.9, multiplied with the gain of V1A. This number should, according to my thinking, be equal to the required grid swing on the input of my power stage. (200mV input * 5 * gain of V1A * 0.9 = Vg) or (1 * gain of V1A * 0.9 = Vg) right?

What I experienced to be confusing: when calculating the voltage drop across the plate resistor, should you use the idle current as a starting point or the current swing (magnitude of the amplified current)? I just took the idle current of 16.5mA as you can see, 1.1V bias, 90V plate voltage, so that leaves 91.1V on the low side of R10 and B+ on the high side. But what I don't understand: The low side of R10 is connected -more or less directly- to the grid of V1A. Does this imply that the grid voltage of V1A will be 91.1V? Or am I wrong referring it to ground?
If figured the push-pull stage always has to be working in Class B. Am I correct? Or can a push-pull stage run in Class A as well? Anyhow, if it has to be Class B, I'd have to set the idle current up a bit right? Or is my configuration of the power stage already good? I haven't touched that part

Furthermore, is it just me or are my resistor values very low? Everywhere I look, I see values between 1k-100K and i've got 68 ohms.

As for the powersupply and R19: If all the tubes are run on 90V, how can i decouple the powersupply rail? Should i just put some caps between each stage and no resistors or do I have to do it in another way? Again, the story about the voltage swing across the plate resistor comes to mind, which I don't really get yet..

I'm really excited now, can't stop drawing and calculating and I have to work tomorrow in about 6 hours Thnx for the help so far

By the way I'm aiming for about 2-5W output power.. So if it can work using a 160VDC powersupply it'd be greatly appreciated


Drawing
  Reply With Quote
Old 17th March 2006, 03:41 AM   #8
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
smsmonster, R6,7,8,9 are not a divider network, R6,7 are grid stopper resistors and can be ignored (not omitted). To know how much swing to provide for the output, bias the output stage, and look at the voltage across R's 11,12,13,14. The voltages should all be the same. You should be able to swing at least 4-8 times this IMO. ie, up to zero, and back down to twice the voltage, plus 6-12dB for good measure (more is good).

As I have said, I am not familiar with e88cc but I know that 16mA is way more than necessary here (but it might be perfect for this device), thing is, a large plate resistor can help you get higher gain if desired.

Quote:
But what I don't understand: The low side of R10 is connected -more or less directly- to the grid of V1A. Does this imply that the grid voltage of V1A will be 91.1V? Or am I wrong referring it to ground?
You need a coupling capacitor. Without one, you are absolutely correct about your assumption. The stage 'idles' at a certain quiescent point. This current is DC. The signal is AC, it 'rides' on the DC so the _average_ current in the stage is always the DC. Since capacitors block DC not AC, you can set the DC for each stage separately and still feed the signal through.

Best to use the manufacturers recommended application as a starting point for biasing your output stage. You should get a hold of http://www.duncanamps.com/tdslpe/tdslpe_install.exe if you don't already have it. Plenty of info plus links to the data sheets and manufacturers recommended applications.

Class B is where one 'side' of the PP amplifies one half of the signal. It is standard to overlap slightly (idle both sides up slightly) as this gives the best results. This is called class AB. Class B is uncommon in audio but we just call it class B for simplicity. Once you get going, you can bias the stage up to full class A if you want to.

The 68 ohm resistors do look unusual. but according to TDSL, 15mA current, Va=90V, Vg=1.2V so it looks like you've done your homework. Thing is, as I mentioned, 16mA is higher than is common for a pre stage.

As far as decoupling is concerned, The earliest stages need less swing anyway, and you can use less than 90V on the plate if you wish. Do use resistors with the caps.

Good to hear you are doing well.
  Reply With Quote
Old 17th March 2006, 11:49 AM   #9
diyAudio Member
 
smsmonster's Avatar
 
Join Date: Mar 2006
Location: NL
Hey, I'm at work now so I can't upload a schematic but i have been on it during my lunch break

So far, i've concluded that the gain of V1A should be 5.5x. This because of the bias current on V2A will be around 1mA (class B). To achieve this, Vg has to be -9V. Therefore, the cathode resistor should be 9k if I'm correct.
In order to have 16mA of current through V2A, the voltage on the grid has to go to -4V. This means there has to be a swing of 5V on the grid of V2A? (of course this same story applies to V2B, V3A and V3B)

Then: if the input signal's magnitude is 1V, and the gain should be 5.5*0.9, which is almost 5x or 5*1 voltage swing.

If the gain is to be 5.5x then Ra must be 520ohm because
Gain = (mu*Ra)/(Ra+rp) = (33*520)/(520+2600) = 5.5x

As for the power supply: as 3*16.5mA flows through V1 and V4, and the voltage swing has to be (4*9 = 36)V, B+ must be 90+36 = 126V. Therefore, R19 becomes (156-126)/(3*0.0165)=606 ohms (in which 2 Watt is dissipated).

Are my calculations correct? And is there something dangerous about my design? Should I put another resistor between V4 and V1 in the B+ line? Thanks a lot
  Reply With Quote
Old 17th March 2006, 12:29 PM   #10
lndm is offline lndm  Australia
diyAudio Member
 
Join Date: Mar 2006
Location: nsw
I think I'll hold off till you can supply a schematic of where you are up to, I may be able to help more then.

A few points. The plate resistor on V1A. For starters, gain is lower than expected. You need to use a plate resistor at least twice the valve's plate resistance (rp) which with your operating points is around 2,650 ohms. You should really go for three times rp or higher. I think Your plate resistor should be at least 10k, and I think others use 25k. The higher values will provide less distortion. If this is a problem with supply voltage, reduce the current from 15mA. Try to set the plate at around half the supply voltage. This suggests 180V supply, say for eg, 15k plate resistor and 6mA current. Feel free to up the supply voltage if desired.

All this is good as you will want more than 5v swing to drive your output stage. Don't underestimate a person's tendency to push a low powered amp. It is essential that it sounds ok when you do. First thing is to make sure the driver stage doesn't stress.

And yes, I feel you should put an RC before V4 (decouple the supply rail). Not only is it necessary, but it will lower the hum (as v4 is amplifying low level signals).
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Zoom H2 modification; now with 4 line inputs Bertoa Everything Else 6 15th June 2013 10:22 PM
Multi-purpose Power Amplifier domtw Chip Amps 22 23rd March 2009 03:18 PM
what's the purpose of pins 3 and 7 in an 8pin VR tube? jarthel Tubes / Valves 7 9th May 2007 02:42 PM
Multi purpose multi channel gainclone PCB: Team project - group buy rick57 Chip Amps 14 29th March 2003 06:54 PM


New To Site? Need Help?

All times are GMT. The time now is 05:56 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2