Problem-solving for an early '50s tweed build?

[gabasa]

Hi Everyone,

I luckily scored a pair of full-sized chassis that can house octal Super/Pro tweed builds. I had a separate thread going that turned into a conversation about these potential builds, so I thought I'd start a new thread to keep things on-topic.

These chassis are punched for octal preamp tubes, so I can either use NOS 6SC7, NOS 6SL7, current Sovtek 6SL7, or noval tubes with chassis adapters. I'm thinking of doing 5D4/5D5 types of builds, but my main concern is preamp tube choice and availability. If I do octal builds, it would be incredibly difficult to find NOS non-microphonic 6SC7s that test well without spending a small fortune, unless someone here knows of a source.

I've read that the curent Sovtek 6SL7 preamp tubes have much less issues with microphonics, but it's hard to find much info on how they sound in comparison to NOS. Otherwise, I can use 5751s all around, or 12AY7/12AX7. Any advice would be appreciated, and as always, thanks in advance.

 

[Wyatt]

Teh current Russian 6SL7 is popular in the hi-fi world. Is it as good or the same as the NOS? That doesn't matter to me as long as they sound good. People have been building some 5B6 clones with Russian 6SC7 and 6SL7 equivalents and seem happy. I would roll the dice and go octal, they do offer a certain "fullness" to their sound. You can always switch later.

 

[gabasa]

Teh current Russian 6SL7 is popular in the hi-fi world. Is it as good or the same as the NOS? That doesn't matter to me as long as they sound good. People have been building some 5B6 clones with Russian 6SC7 and 6SL7 equivalents and seem happy. I would roll the dice and go octal, they do offer a certain "fullness" to their sound. You can always switch later.

Thanks, I didn't know that there are current Russian 6SC7 tubes, I thought they only made 6SL7. That's helpful.

 

[printer2]

Almost would want to go loctal, pick up a few spares, wonder how many they have? Entered ten and the shopping cart took it.

7F7 Loktal Twin Triode Tube

New old stock loktal twin triode. Various brands. Essentially a loktal 6SL7GT. NSN/FSN = 5960001883549, 5960006155587 Signal Corps Number = VT-189

vacuumtubesinc.com

Really just kidding though.

 

[gabasa]

Almost would want to go loctal, pick up a few spares, wonder how many they have? Entered ten and the shopping cart took it.

7F7 Loktal Twin Triode Tube

New old stock loktal twin triode. Various brands. Essentially a loktal 6SL7GT. NSN/FSN = 5960001883549, 5960006155587 Signal Corps Number = VT-189

vacuumtubesinc.com

Really just kidding though.

Wow, I've never heard of a local tube before. I thought that was a typo at first!

 

[printer2]

Wow, I've never heard of a local tube before. I thought that was a typo at first!

So does Google. Actually found an inexpensive number of tubes, do I need an amp with them rather than octals? I am going to have to go through my octals to see if I have enough for an amp. I keep forgetting you are in Canada, shipping tacks on a bit to the price of a tube.

 

[gabasa]

So does Google. Actually found an inexpensive number of tubes, do I need an amp with them rather than octals? I am going to have to go through my octals to see if I have enough for an amp. I keep forgetting you are in Canada, shipping tacks on a bit to the price of a tube.

Thank you.

 

[Wyatt]

Wow, I've never heard of a local tube before. I thought that was a typo at first!

Since they have a proprietary socket, not a lot of demand for Loctal tubes in the consumer market, so they can be a bargain if you build around them.

Another good option, but requires the right filament supply. are 12.6-volt tubes. A NOS RCA 12AC7 is the exact same tube as their 6SC7 but with an 12-volt heater...and they only cost $6. The 12V6GT is the sister tube to the 6V6GT black plate, but NOS RCA only cost $10/ea.

 

[gabasa]

A question just popped into my head regarding the use of octal tubes, because I’m determined to use them in at least one build. If I take the 5D5 circuit, which I posted above, could I simply substitute the preamp tubes for octals? I’m thinking of using a pair of 6SC7 on the inputs and 6SL7 in the phase inverter. What I’m not sure about is if any adjustments need to be made to any of the voltages anywhere. Can these octal tubes operate exactly the same as the 12AY7 and 12AX7 that are in the original schematic? I hope my question makes sense, thanks.

 

[Huddy]

Why not follow the 5c5/5b5 schematic for octals? They're pretty much the same as 5d5 except with octals. All 6sc7s. 5B6 bassman uses a 6sl7 phase inverter which pretty close to a 6sc7 except each triode has its own cathode.

 

[Tele-Mania]

Don't forget to add grid stopper resistors and 470ohm screen resistors to the output tubes for better stability and best practice. These old amp designs are very primitive and leave some room for improvement. See the following article. https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

 

[printer2]

A question just popped into my head regarding the use of octal tubes, because I’m determined to use them in at least one build. If I take the 5D5 circuit, which I posted above, could I simply substitute the preamp tubes for octals? I’m thinking of using a pair of 6SC7 on the inputs and 6SL7 in the phase inverter. What I’m not sure about is if any adjustments need to be made to any of the voltages anywhere. Can these octal tubes operate exactly the same as the 12AY7 and 12AX7 that are in the original schematic? I hope my question makes sense, thanks.

Since they have a proprietary socket, not a lot of demand for Loctal tubes in the consumer market, so they can be a bargain if you build around them.

Another good option, but requires the right filament supply. are 12.6-volt tubes. A NOS RCA 12AC7 is the exact same tube as their 6SC7 but with an 12-volt heater...and they only cost $6. The 12V6GT is the sister tube to the 6V6GT black plate, but NOS RCA only cost $10/ea.

12SC7, probably hit the A as it is beside the S.

I have way too many 12V6's. The loctal prices cab be less than the 12V versions of the tubes. If I were to build an octal amp for myself I probably would go that way given what I know now. Mind you, you better get spares as the corner music store will not have them.

 

[Wyatt]

12SC7, probably hit the A as it is beside the S.

Yep, but I can't edit it now.

 

[gabasa]

Why not follow the 5c5/5b5 schematic for octals? They're pretty much the same as 5d5 except with octals. All 6sc7s. 5B6 bassman uses a 6sl7 phase inverter which pretty close to a 6sc7 except each triode has its own cathode.

I'd love to try the 5D5 circuit because the inputs are wired a bit differently, and it has the split cathode paraphase PI. Just wondering if the 6SC7 and 6SL7 could be used as direct substitutes for the 12AY7 and 12AX7. I realize that the gains are a bit different, but besides that. From what I can tell, it should work fine.

 

[2L man]

I'd love to try the 5D5 circuit because the inputs are wired a bit differently, and it has the split cathode paraphase PI. Just wondering if the 6SC7 and 6SL7 could be used as direct substitutes for the 12AY7 and 12AX7. I realize that the gains are a bit different, but besides that. From what I can tell, it should work fine.

When tube itself has higher gain it is easy to tune stage that amplification comes lower but other way it does not work. Sometimes tubes output impedance limit the amlification and following stage must be changed or tube changed. I would say if pedals are used their output amplify guitar signal so much that amplifier tube gains significance comes less important.

I have planned and prepared to install a reverb pedal circuit board, which do not have any use in effect board anymore, in between amp circuit where it can function as a gain stage and to produce the reverb which I seem to use most of the time anyway.

Tube datasheets often have example circuits which can be compared. Also using Universal Loadline Calculator is handy for testing dufferent Operating Points

 

[NTC]

And don't forget the handy tables of standard circuit values in the back of RCA tube data books. They can help with providing initial comparisons between tube types.

Before I knew any better, I set up a breadboard circuit with typical 12AX7 values (100k anode load, 1.5k cathode resistor) and it worked fine with a 6SL7. You usually see something like 220k or 270k on the plate and a 2.2k on the cathode with a 6SL7. And note some of those old tweed schematics where they state that they just subbed in 12AY7 and 12AX7 tubes for the original 6SC7's.

 

[peteb]

Why not follow the 5c5/5b5 schematic for octals? They're pretty much the same as 5d5 except with octals.

this is what I was thinking too.

I'd love to try the 5D5 circuit because the inputs are wired a bit differently, and it has the split cathode paraphase PI.

the input of the 5C5 is grid leak bias while the input of the 5D5 is the more typical cathode bias.

the decision needs to be made whether you want the sound of grid leak bias or cathode bias. Grid leak breaks up earlier and is more lo-fi.

I don’t see any differences between the 5C5 inputs. They all look the same to me. I see the interactive tone control like the 5E3, that looks to be on one side but affects all inputs.

likewise, I don’t see any difference between the different inputs on the 5D5. The 5D5 also has the same interactive tone and volume controls.

there is the possibility of one grid leak biased input tube and one cathode biased input tube.

 

[gabasa]

And don't forget the handy tables of standard circuit values in the back of RCA tube data books. They can help with providing initial comparisons between tube types.

Before I knew any better, I set up a breadboard circuit with typical 12AX7 values (100k anode load, 1.5k cathode resistor) and it worked fine with a 6SL7. You usually see something like 220k or 270k on the plate and a 2.2k on the cathode with a 6SL7. And note some of those old tweed schematics where they state that they just subbed in 12AY7 and 12AX7 tubes for the original 6SC7's.

That's exactly how I see it. If I want to ditch the grid-leak bias and use a split-cathode paraphase PI, that's all in the 5D5 schematic. The only decision I'd have to make is of plate and cathode resistor values, and based on what you've said, either should work.

 

[gabasa]

this is what I was thinking too.


the input of the 5C5 is grid leak bias while the input of the 5D5 is the more typical cathode bias.

the decision needs to be made whether you want the sound of grid leak bias or cathode bias. Grid leak breaks up earlier and is more lo-fi.

I don’t see any differences between the 5C5 inputs. They all look the same to me. I see the interactive tone control like the 5E3, that looks to be on one side but affects all inputs.

likewise, I don’t see any difference between the different inputs on the 5D5. The 5D5 also has the same interactive tone and volume controls.

there is the possibility of one grid leak biased input tube and one cathode biased input tube.

I've read that PAFs can distort too much with grid leak bias.

It looks like the typical 5E3 volume/tone configuration to me, with bright and dark channels, but I get what you say. There are no high and low inputs from what I can see.

This will be interesting. The seed was planted last year for an octal build and regardless of how it happens, I'm excited to try it. Apparently they do sound different that 12AY7 and 12AX7.

 

[gabasa]

After watching this video, I think I'm hooked on the idea of building a '40s circuit. It sounds more lo-fi than the '50s models, but in such a cool way.

6SC7's, I'm coming to get ya!