New Project - Bassman Lite Octal

[Phil_S]

Now that I've been able to wring out the power supply in that RCA PA I was given, I'm determined to build a useable and toneful guitar amp into the box. In addition to the rectifier, there are 6 octal sockets and 2 magnoval that must have been used as plugs, not for tubes. There are 4 500K pots mounted on the front panel plus an on/off switch. I'm not looking to make any new holes, so this becomes a limiting factor. I've got the original knobs and I'd like to keep it looking mostly original. I also have the cage and the bottom plate! I've already wired a 3-prong cord and was able to re-use the strain relief grommet. I will need to build the preamp, PI, and bias supply.

Due to the high B+ (Va=460), I'm thinking I should stick with 6L6GC for power tubes. With 4 pots, I am looking at a TMB tonestack plus one for volume control. I expect to change the pots but the problem there is the D shaped fitting for the knobs...not sure what I'll do here. I don't think for a minute I can file down the shaft of a typical pot.

For the octal tube compliment:
V1 6SL7
V2 6AQ7 (only one triode needed)
V3 6SQ7 (PI, got a design from Aiken's pages)

I've drawn a schematic, attached. Sorry it turned out so small, but you can blow it up if you are interested.

All comments are appreciated, but I do have a few questions and would be grateful for some input.

What happens to the tonestack and volume control if I use 500K pots instead of 250K for the Treble and Bass and instead of 1M for Volume? This would solve a big "problem." I'd omit the Mid control and leave one knob non functional.

Is 820 the appropriate value for NFB from the 4 ohm tap?

The combination of the choke and the big resistor are dropping 80V. Tonally, what should I expect by dropping the screens so much. The choke only drops 1V by itself. In it's present state, the screens are pulling 2mA each.

I expect to tweak the power supply resistors on the fly, as well as the bias supply dropping resistor.

Any other general critique of the schematic is welcome, including such things as choice of cathode resistors and coupling caps.

As always, I look forward to what other have to say.

Phil

[tictac]

You have a Voxish PI with a Fenderish Negative Feedback network...

Not that it won't work but maybe raise the 820R Feedback resistor...

Also that's an awfully large value screen resistor; your going to really throttle back your power amp if you leave that as is. Why such a large value? Or why not try no resistor at all?

[Phil_S]

You have a Voxish PI with a Fenderish Negative Feedback network...

Not that it won't work but maybe raise the 820R Feedback resistor...

Also that's an awfully large value screen resistor; your going to really throttle back your power amp if you leave that as is. Why such a large value? Or why not try no resistor at all?

Thanks for the insights. I'm an amateur. There is much I don't understand so well. It sounds like I should look at the more blackface-ish TB tone stack. I'll think about this.

As for the screen R, it is here: https://tubeamparchive.com/viewtopic.php?t=22991 I chose the 20K because I wanted to get the screen voltage down from 460-ish and that's what I tested the power supply with. You can see there are three lugs. There is 3K between two close together and 20K between the two far apart. I think I need to move the wires to the 3K!

Thanks very much!

[matt h]

(deleted)

[Phil_S]

Hi Matt. I'm not offended at all! I figure it's better to start the discussion somewhere than nowhere. It gets people to react. I think the Rockster into 6L6's is a splendid idea! (Kinda stuck with the OT...not a bad thing really.) I hadn't thought of it. I'll have to study the schematic and see if I can do it with this donor iron. I also hadn't thought about the benefits of cathode bias with respect to plate voltage. Good suggestion all around!

Do we know if this was ever finalized?
https://tubeamparchive.com/download/file.php?id=7748

OK, I delayed pressing the "send" button to do a little reading. I think this may be something to get excited about!

This will not be the "low voltage" Rocket or Rockster. This will be the Monster Octal Rockster. I can't really control the B+, which will be around 460. (Get the right tranny or work with what you have!) So, working in recycling -- the mother of invention. I'll have to dump 200V to get the Rocket preamp in the relevant range. That might be a tall order. That's the spot for the honking big 23K resistor in the B+ ladder! Maybe I just build it and see what it does?

So, assuming plate voltage around 460 for a pair of 6L6GC, what would you like to see for the screens? The rest I can probably calculate and produce.

Is this nuts or what?

[matt h]

(deleted)

[Phil_S]

Wow Matt, that's a lot to think about. I'm not sold on any particular tubes, but I am wanting to work with the existing octal sockets and more importantly the existing iron. I prefer to keep the budget down on a project like this as it is more for my own amusement than anything else. As I may have said earlier, a 50W amp isn't something I'll see as a keeper. As it is a 30W amp is too loud for my needs.

If I've got to change the transformers, this project loses steam rather quickly. The OT primary is about 6600 ohms. The good news is that it has 4-8-16 ohm outputs, so I suppose it can also be 3300:2-4-8 allowing for EL34's. Unfortunately the only speaker I've got that will handle a 50W amp is 16 ohms...pause for the thought...I can always keep the 6L6's.

Either way, I'll have to buy some parts, so that makes time to consider the various possibilities. Here are the test voltages (without the 20K resistor in use) from my power supply build out to the power tubes (I suppose they will vary a bit with line voltage of the day), cathode biased with 282 ohms (actual meter reading) for Rk.

HT supply 819VAC.
B+1 449V, Plates 448V
(choke between B+1 and B+2)
B+2 449V, Screens 447 or 448 ... the meter was waffling
Vk 37.7V
Calculations give me:
Ia 132mA
Ik 134mA
Plate dissipation 54W

I will take to heart what you say about screen voltage and see if I can lose the unreasonable fear I have. I am unaccustomed to working voltages this high -- that's what's driving me to say some of these things.

On the choke, unfortunately the donor amp came without a schematic and one is nowhere to be found. I could not understand the circuit as-built and I suspect it was incompetently tampered with as well, particularly with respect to how the choke was used. Size-wise, it is not as large as the 20H 100mA Vox AC-30 choke I've got on the "bone pile" but it is still sizable, approximately 3.1"h x 2.5"w x .75" stack depth. The core is roughly 2" x 2.5". I'd guess it is at least 5H @ 75mA, certainly up to the job as a screen choke.

I was wondering about Aiken's choice of the tail resistor. As you probably know, he makes a persuasive case for everything he writes. Aiken offers a brief discussion of the tail resistor suggesting something smaller might be more to taste:
Now, the last thing to determine is the value of the "tail" resistor. Since we decided to raise the cathode by around 80V, and the total cathode current is 2*0.86mA = 1.72mA, the required tail resistor is 80V/1.72mA = 47K. This is a relatively large value in comparison to the bias resistor value of 680 ohms, so it should approximate a constant current source well enough for our needs. As mentioned earlier, if this resistor is too large, the headroom, i.e. available output voltage swing, will be too low. The reason for this can be seen from the above plate curves, by reducing the plate voltage and redrawing the load line. As the available plate-to-cathode voltage is lowered, the maximum available output voltage swing is reduced. http://www.aikenamps.com/LongTailPairDesign.htm

On the PI, I stuck with the 82K plate load resistors for the 6SL7 also based on Aiken. I don't understand the balancing discussion very well. I can take voltage measurements and experiment if needed. Curiously, he sticks with the 1M grid leaks and his choice of Rk = 680 seems good to me.

For the preamp tubes, for 6SL7's I'll need to dump some voltage no matter what. RC-30 says 300V max and typical is 250V. If I'm working with 450 at the screens, that's 200V above what I need. Well, yes, I know, there is a differential between B+ and plate voltage, so maybe I only need to dump 150V, but I think the general idea is solid.

[xtian]

Guys, I've been reading with interest. Did you see my thread on Mitchell Pro-100? I have a quad of Electro Harmonix 6L6GC with 483 on the plates. I have 470R screen resistors, and a choke between OT CT and screens, but I can't detect any voltage drop on the screens compared to the plates, given the resolution of my voltmeter! The voltage drop across the screen resistors is about 6.5v, which means about 14mA! Can this be right? Amp sure seems to be running nicely. Quiet and loud when appropriate.

[Phil_S]

According to Duncan's TDSL, in AB1, Va 450, Vg2 400, Ig2 is 5.6-22.0mA. So, 14mA seems OK to me. The new Russian 6p3c tubes I'm using are only pulling 4mA on the screens (that's 2mA each.) Of course, the amp isn't built, only the power section for testing.

Remember, a choke is not static like a resistor. It reacts to the load.

[matt h]

(deleted)

[Phil_S]

Interesting...let me see if I can organize this discussion and boil it down a bit.

I think we've settled the power section discussion, a pair of cathode biased 6L6GC. We're going to try to work with the B+ the existing iron produces. The backup plan is to dump some voltage with a Zener on the CT. BTW, I have plenty of real estate in that part of the chassis to heat sink a Zener if I can find the right shaped heat sink on my bone pile.

PI, also basically settled except for the tail resistor and the question of balance and NFB. This should shake itself out quickly...something between 15K and 47K for the tail. I like the ideal of starting with 47K and tacking in another in parallel. I'm not looking to booger the chassis with too many new holes...trying to avoid switches. That's not to say I won't do it. The questions of NFB and balance -- plate load resistors and NFB resistor can easily be modified.

Preamp...users choice? I have to admit I'm a little bit embarrassed for giving bad info. There are 3, yes 3, preamp sockets (picture) plus the PI socket! I think this is where we've got to consider the limiting factors. Now, my head is starting to swim.

There are 4x 500K pots with D shafts. The 5th hole is a rotary on/off. I suppose I can pull that for a 5th pot and put it somewhere on the back near the PT, but not inclined to do so -- actually looking at the amp, I can probably remove the service outlet on the back and replace it with a rocker on/off, which might actually be an improvement -- I'll have to see about fitting it. I have the original knobs. I'd like to keep them and I'll need real convincing to make new holes in the front -- don't wanna do it. I don't mind changing the pots, but where to get D shaft pots without spending an arm and a leg? I think I'm good with Gain, Treble, Bass, and Master, except that I need a dual pot with a D shaft for the Master! I'd be OK with a single tone knob

Input jacks will be from the left side near the preamp sockets. I've got to patch the holes where the 9 pin plugs were removed.

I think the monster 23K needs to stay put. It is nicely mounted.

The vertical shield plate between the pre sockets and the power sockets can be removed but I'm thinking it's not really in the way for access to the PI socket.

The use of the can cap is temporary. That is a 3 section 40-40-40 and it's old. I'm waiting for some 47uf 400V singles to make a 23uf 800V for the first node and then 20uf or 10uf singles for the rest.

The build is likely to be a hybrid PTP, terminal strip, and turret board. I'm inclined to put as much as I can on the pots and tube sockets. I've got a couple of open areas for boards -- about 4" x 3" between the OT and the pilot, and about 5" x 3" in the corner defined by the pilot and the 23K (where you see the hanging b/w twisted pair that will be clipped.) I'm thinking the 5x3 area is less useful in terms of location.

There is a bit of space on the back panel to put a few things. I'm wanting 3 output jacks 4-8-16 -- that's the easy part. If there is anything else, layout consideration will be the challenge. Take a look. I'll be removing the phenolic plate with the output screw, and I can remove the remote speaker plugs. The lower left is too close to the power tubes and space is tight. The two holes marked "speaker" might be a bit close to the rectifier and will be distant from connection points. Well, all in good time I'm sure a layout will take shape. This isn't my first rehab job.

Thanks again for the conversation. It's very helpful.

[ampfab]

as far as the 500k pots, i would attempt to use the existing 500k pot for volume. based on previous experiments of mine, probably not much of a noticable difference between 500k and 1M.
in times of need, ive "converted" 500k pots to approximately 250k pots by adding a 470k resistor in parrallel with the pot (just connect the resistor to the outside terminals of the pot) this seems to work okay in tone controls, not so well in volume controls.

[surfsup]

Guys, I've been reading with interest. Did you see my thread on Mitchell Pro-100? I have a quad of Electro Harmonix 6L6GC with 483 on the plates. I have 470R screen resistors, and a choke between OT CT and screens, but I can't detect any voltage drop on the screens compared to the plates, given the resolution of my voltmeter! The voltage drop across the screen resistors is about 6.5v, which means about 14mA! Can this be right? Amp sure seems to be running nicely. Quiet and loud when appropriate.

The OT acts as a resistor and drops voltage because it has a resistance. Could be that the voltage drop at idle is the same from:

1) the supply node across OT winding to plate
2) from the supply node across choke across screen resistor to screen

Or maybe I misunderstood your post?

[xtian]

No, I think you understood. Just learned a new axiom, "don't expect the screen voltage to be lower than the plate voltage at idle in a fixed bias amp." Sound right?

[Phil_S]

as far as the 500k pots, i would attempt to use the existing 500k pot for volume. based on previous experiments of mine, probably not much of a noticable difference between 500k and 1M.
in times of need, ive "converted" 500k pots to approximately 250k pots by adding a 470k resistor in parrallel with the pot (just connect the resistor to the outside terminals of the pot) this seems to work okay in tone controls, not so well in volume controls.

I wanted to know about this. In particular I was wondering about a resistor across the outer lugs, and/or ~250K resistors on the outer lugs to make a 1M. Thank you for sharing your experience. It sounds like I don't need to bother much with it and this is good news. I'm going to try to use the existing pots. Besides, some of them meter in the low 400's so they make better candidates where a 250K is needed. If it doesn't work out, there's always Plan B. Hmmm...I don't really have a Plan B!

In the meantime, it's a slow say work-wise, so I've been tinkering. I realized that I could remove the convenience outlet on the back and insert a rocker on/off, which I did. This accomplishes two things.
1) It moves the on/off very close to the fuse and 120VAC doesn't need to run around the edge to the front panel. I like this dress better.
2) It frees the rotary on/off to be used in the tone stack as a lift switch. This is a bonus...I can (for low DC voltage only), for example, lift a parallel resistor or maybe use it as a bypass of some sort. It opens up possibilities. Bright switch, 2-setting Mid control (seems like a wasted opportunity), something else? I'm sure it will get used.