| rvas18 |
I am building my first amp soon and I have been working with my schematic. I know that the 6v6 and the 6bq5 tubes are similar in many ways but I am not knowledgeable enough to modify the values on the schematic myself. I modified the original by removing the feedback, I added a self-biasing cap to the 12ax7, and I think I correctly added a switch to go between triode and pentode mode (the original was drawn for pentode mode). It is necessary that I use the 6bq5 and the 12ax7 because I already have a good number of each tube on hand. The transformers were salvaged from an old RCA SE console unit. I would appreciate any help on turning this schematic to accommodate a 6bq5 tube, and also if possible suggestions on where and what parts to get for the coupling caps, resistors, and bias caps. I am not completely set on using this schematic either, if ananyone knows of a 12AX7, 6BQ5 SE amp that will obviously outperform the one I am trying to configure please post a schematic.
Thanks,
Raśl
Shematic Link |
|
|
| fdegrove |
Hi,
Hi,
The EL84/6BQ5 is a drop in replacement for its granddaddy, the 6V6.
Once you have the noval socket in place just check the pinout changes if any, (I didn't check) and it should work.
Cheers, ;) |
|
|
| Sparky OR |
| I've usually seen higher value cathode resistors for the 6V6 than for 6BQ5 in SE. Can't see your schematic from the link, as you apparently to be a member. |
|
|
| Sch3mat1c |
| quote: | Originally posted by fdegrove
Hi,
Hi,
The EL84/6BQ5 is a drop in replacement for its granddaddy, the 6V6.
Once you have the noval socket in place just check the pinout changes if any, (I didn't check) and it should work.
Cheers, ;) |
Erm... maybe if you drop the cathode bias by about half... but then, that's obviously not plug-in... It's got a lot more Gm.
Tim |
|
|
| bournville |
Its true that a 6BQ5 is a 'beefier' tube than the 6V6, and ideally it should have a resistor for cathode bias of about 150 ohms as opposed to the 240 ohms in the circuit diagram, but it will work in this circuit as a direct replacement - you could argue that the higher cathode resistor will reduce the gain/output of the 6BQ5 to nearer the levels of a 6V6. Since in optimum conditions 6BQ5s take a higher HT current you might not want it in such conditions, as it increases demand on the power supply - the extra heater current will be putting greater demand on the power supply anyway.
On a couple of occasions in old tube sets I've replaced the 7C5 (the loctal equivalent of the 6V6) with a new socket and a 6BQ5 without making any other circuit alterations (apart from changing the dial lamps for ones with a lower current rating to reduce demand on the mains tranny) and they work OK - it theory it may be bad practice but in reality there is no problem.
With the circuit given, one addition I would make would be to add a small cap across the primary of the output trannies to reduce third harmonic high frequency transients when run in pentode/tetrode mode, as there is no negative feedback to reduce this. |
|
|
| rvas18 |
I have taking many of the suggentions i have gotten and reworked the schematic onve again. I have lowered the cathode resistor to 150ohms, and implemented a swithced on\off negative feedback, and decreased the input pot from 500ohms to 100ohms. Because this is my first amp i would like to be able to use try all the different configs (no NFB\NFB, triode\pentode, etc) without having to build several different amps.
i have another possibly quick question, a friend of my dad gave me some more vintage tubes, i got 2x 12h6, 4x 12SL7GTA, 6x 12AU7A, 2x 6AQ5, 6x 6X4, and 6 more 12AX7A's are any of those useful for audio purposes, and if any are can they make a higher quality amp than the 12AX7\6BQ5 amp i am configuring now? |
|
|
| bournville |
I don't think that the 6AQ5s would give you any advantage over the 6V6s you started with in the circuit you are using.
With the second schematic you've posted, I assume that the 150 ohms cathode resistors in the output stages should be for the 6BQ5s and not the 6V6s? Also, the 330mfd caps across the cathode resistors in the 12AX7 stages would negate the effectiveness of the negative feedback arrangements. |
|
|
| battradio |
| The 6BQ5'S will draw almost twice the fillament current as the 6V6'S ,if the transformer powered other tubes in the stereo it will be ok ,but if only powered the tubes shown you might want a seperate fillament transformer . |
|
|
| rvas18 |
| i plan to use the 6v6s now |
|
|
| Sch3mat1c |
Ah, good point, NFB will indeed be shunted. Best way to apply it anyway is to add 100 ohms or so, below that bypass cap, and connect NFB there. The resistor will also have to be smaller, maybe 2k (vs. 20k as shown).
Yes, 6V6s typically use around 470 ohms in this circuit. Even 6L6s in a similar circuit would run 330 ohms.... come to think of it, a 'BQ should only run as much too. 240 sounds a little close now that I think of it...
Tim |
|
|
| rvas18 |
| quote: | Originally posted by Sch3mat1c
Ah, good point, NFB will indeed be shunted. Best way to apply it anyway is to add 100 ohms or so, below that bypass cap, and connect NFB there. The resistor will also have to be smaller, maybe 2k (vs. 20k as shown).
|
can some one possibly take the time to show me how to do this so the amp will work well in both pentode and triode, maybe just a quick msPaint scribble and an email to me at: rvas18 (at) hotmail dot com . i am completely new to tubes and i am learning a lot as i go. i have breadboarded and designed PCBs for solid state amps and such but never tried to customize a schematic of a medium so foriegn to me. all the help you guys here have given and are willing to give is incredibly appreciated.
thank you once again,
Raśl - rvas18 |
|
|
| rvas18 |
| hey, tom what is the purpose of that unmarked cap on the feedback resistor? |
|
|
| Sch3mat1c |
Reduces HF gain and causes a phase shift which tends to cancel with that caused primarily by the OPT. So it stabilizes it a bit. Tends to square up the corners of a square wave, or is it reduce overshoot? I forget.
Tim |
|
|
| rvas18 |
is there any particular value i should make it that capacitor?
can i substitute the 2.2k for a 2.7k resistor at the cathode or will that cause a mess of changes? aslo if i turn off the feedback will that resulting circuit run decently for triod mode? |
|
|
| Sch3mat1c |
For proper application of NFB you really need a good understanding of what's going on, a scope and a function generator. For now you could use a 12AU7 (instead of the 'AX) and cut the 2.7k+cap out of the circuit, and likewise short the now-extra 220 ohm cathode resistor to ground.
Come to think of it, a 1k would be better for the 12AU7 cathode. And 22uF across it.
Tim |
|
|
| fdegrove |
Hi,
| quote: | | Tends to square up the corners of a square wave, or is it reduce overshoot? I forget. |
The latter.
| quote: | | Come to think of it, a 1k would be better for the 12AU7 cathode. And 22uF across it. |
1K + at least 47µF across it...
Otherwise you can also lose the decoupling cap and relax overal NFB.
Cheers,;) |
|
|
| rvas18 |
| so is my newest revision of my schematic wrong?? |
|
|
| rvas18 |
| also how do the 12au7 and 12ax7 compare. and is it hard to use a scope and function generator because i live at a math and science high and our physics department has both pieces of equipment which i can borrow if needed. |
|
|
| Tom Bavis |
The 2.7K cathode resistor was used because I had a whole bunch of 150K resistors and was running low on 100Ks... so the higher plate load means less current, and the 2.7K gave it a better operating point. With 100K plate load, 2.2K is probably better.
As for the feedback cap, it hasn't been installed. After a few other projects get done, I may hook up the audio generator and 'scope and optimize it. With feedback the amp is flat from 40 to 20,000 at low power, though there's a peak at 40 KHz, which that cap would probably tame. 270 pF is a typical value, but something else in the range 50-470 might be better.
If you have access to a 'scope and signal generator, it would be good to hook it up and take a look. |
|
|
| rvas18 |
| my problem is i dont know what to look for on a scope. |
|
|
| rvas18 |
| tom, you used a 2.2k in your schematic on the cathode then a 220 to ground. you used a 150 on the plate, and a 2.7k on the feedback. also when you say you didnt use a feed back cap that is the unmarked one, right? and are the values on the 12ax7 arent affected because of the differences in the 6bq5 and 6V6? |
|
|
| Tom Bavis |
You're right - I should look at my own schematics... and as you thought, the voltage amp stage isn't affected by the choice of output tube.
The operating point of the first tube is set by the plate resistor and the cathode resistor (2.2K and 220 in series = 2420 Ohms)
The feedback resistor (2.7K) value can be adjusted to set the overall gain of the amp - higher gives more gain, also higher output impedance, so speaker variations will have more effect. And distortion goes up with less feedback - less of an issue in triode mode, which has lower output impedance and lower distortion. Obviously the feedback cap value isn't too critical, if I can leave it out completely. |
|
|
| rvas18 |
| will it hurt to increase the 4.7uF cap to 22uF or even 47uF. Also you have have a 4.7k resistor in between the pot and the screen, does this chang anything in my situation considering i omitted this??? |
|
|
| Tom Bavis |
| The larger cathode bypass wil lbe fine. The input resistor (often rerred to as a "grid stopper") helps prevent radio frequency interference, but isn't essential, especially if you use an input pot. |
|
|
|
