Bass Amp for a friend

[didit]

As in some full of awe, so doubtless right word would be awesome.

Enjoy.

Best .. Ian

[Colossal]

Cheers Ian. I will post schematics pretty soon. Want to hammer out the details a bit more.

[roberto]

Hi Colossal,

I'm still working too on the design of the bass amp we were talking about.
Have you started to build this amp?

Have a nice day

[JazzGuitarGimp]

This is more a question than a suggestion. I’ve put a bit of thought into this over the past ten years because I am very intrigued by the idea of AB2, but I’ve not done any experimentation to this point. What if you were to use an asymmetrical bipolar supply for the 12AU7 CF drivers? Let’s say you’ve done the load line for the output tubes, and you’ve decided +25V is as far as you want to push the output grids. Why not run the CF at +25V on the plate, and -200V on the cathode load resistor?

[roberto]

Mosfets are better than tubes to drive power tubes in AB2 (see Powerdrive article).
Asymmetric driving is absolutely better in order not to waste energy, with around 3 times the negative voltage and about maximum positive swing + 50 V for the positive side.
EG if you have -52V on 6L6GC's g1 and want to go up to +10V, go for -160V and +60V. Bias voltage is applied to the gate.

[Colossal]

Hi Colossal,

I'm still working too on the design of the bass amp we were talking about.
Have you started to build this amp?

Have a nice day

Hi Roberto,

I am gathering parts. The gentleman that is building the chassis is moving his shop, so I am waiting on that.

[Colossal]

So, getting back around to this build...

I've been doing a bit of reading about the old Ampeg Portaflex designs and found this gentleman's work: https://www.thallenbeck.com/blog/posts/ ... portaflex/

In his design, he tweaked the B15N preamp to use a 6SN7 in place of the 6SL7, citing wanting a reduction in gain and later onset of distortion. In speaking with my friend about his desires for his amp, clean headroom is the goal. He would be using a pedal for distortion, if desired. Also note that the schematic for the amp above reduces the plate load resistors to 100k and 150k respectively (from Ampeg's 470k and 470k on V1a and b). I noticed he flipped the 5k6 cathode resistor on the original schematic from V1b to V1a, and the 2k2 from V1a to V1b. I think the demo sounds pretty darn good (especially with the Jazz bass) and has good clarity and presence.

Another thought:
Any reason not to dedicate a single tube for each gain stage and then run them as parallel triodes? This costs an extra tube, yes, but has the advantage of a little more gain, thicker tone, a little reduction in noise, and a 6SN7 or 6SL7 could be used in either or each position, to taylor the desired gain and distortion characteristic. Heaters will be elevated to DC.

I was considering Aiken's 6SL7 Long Tailed Pair inverter (https://www.aikenamps.com/index.php/des ... e-approach), but any reason not to go with the Portaflex's Paraphase inverter? Following the inverter (no pun intended) is Martin's 12AU7 Cathode Follower driver into an ultralinear output section; that piece is set in stone.

Thanks for any comments and discussion.

[didit]

Hello Dave -

Getting back around to replying...

If "pure clean" is the goal you should most certainly go with CF buffered LTP PI. Having owned an early B15n, I'm confident the paraphase PI is one of it's essential ingredients along with the tone stack and speaker+cabinet. I've built a few other amps with paraphase PI and ascribe a "warmth" to it's inherent distortion. It's not a "tear the face off" burn, but still audible. You might just plan for and build it so you can test out both, and prove to yourself which is best for the friend's goals. I'd tweak gain on 6SL7 before resorting to 6SN7 but that also is something you tweak with before committing to final design. Less than confident giving advice on parallel triode--it works just as you describe but whether all that is necessary seems questionable give how well a stock B15 performs. You might well want to consider using a 6SN7 instead of 12AU7 for the post-PI CF buffer.

One aside--I have on two occasions resorted, with success, to DC filament power on builds around larger 8 pin preamp tubes. Helped with some frustrating heater hum.

Best .. Ian

[roberto]

Any reason not to dedicate a single tube for each gain stage and then run them as parallel triodes? This costs an extra tube, yes, but has the advantage of a little more gain, thicker tone, a little reduction in noise, and a 6SN7 or 6SL7 could be used in either or each position, to taylor the desired gain and distortion characteristic.

Together with the paralleled triodes, there's also the option to have a gain stage plus cathode follower for each stage, to have low output impedance on every stage as well.

Any reason not to go with the Portaflex's Paraphase inverter? Following the inverter (no pun intended) is Martin's 12AU7 Cathode Follower driver into an ultralinear output section; that piece is set in stone.

If you have a 0-4-8-16 Ohm secondary, just connect the 4 Ohm tap to ground, and the 0 and 16 Ohm taps to the cathodes of the output tubes. You will get some cathode feedback together with the screen ultralinear feedback.

[Colossal]

Hello Dave -

Getting back around to replying...

If "pure clean" is the goal you should most certainly go with CF buffered LTP PI. Having owned an early B15n, I'm confident the paraphase PI is one of it's essential ingredients along with the tone stack and speaker+cabinet. I've built a few other amps with paraphase PI and ascribe a "warmth" to it's inherent distortion. It's not a "tear the face off" burn, but still audible. You might just plan for and build it so you can test out both, and prove to yourself which is best for the friend's goals. I'd tweak gain on 6SL7 before resorting to 6SN7 but that also is something you tweak with before committing to final design. Less than confident giving advice on parallel triode--it works just as you describe but whether all that is necessary seems questionable give how well a stock B15 performs. You might well want to consider using a 6SN7 instead of 12AU7 for the post-PI CF buffer.

One aside--I have on two occasions resorted, with success, to DC filament power on builds around larger 8 pin preamp tubes. Helped with some frustrating heater hum.

Best .. Ian

Hello Ian,

Thank you for your thoughtful reply. You've given me a lot to consider. The amp is definitely getting a CF driver post-PI, using Martin's design +/-160VDC for the supply. I will likely just stay with a LTP. I mentioned the B15 as that guy's sort-of clone sounded pretty good and wasn't distorting, and the B-15N is a classic amp. My friend plays a Jazz bass most of the time (he has a Ric also), so his tonal goal is clean, warm, but clear sound with excellent note definition and a lot of presence. I and he and are not fans of a muddy sound where the attack of the note goes "flump". It should be clangy with a lot of apparent depth. The power supply will be plenty stiff with the whole rail seeing a Pi filter. It should punch and stay clear. If you are a fan of Tool, Justin Chancellor (bass) is a great example of this kind of clarity. He has utterly fantastic tone. For distortion, when he needs it, my friend will be using a pedal so it makes sense for the amp to be a good, neutral-ish pallet for that. He plays in a (classic) rock cover band and their drummer is very good, but quite heavy handed. He likes 15" speakers so a pair of Thiele TL606 cabs with either Eminence drivers or, if he can find a decent pair, Electrovoices is what he is thinking.

As for the 6SL7s, I am all for keeping them in the design. Tone stack will be James with a mid-shift. DC heaters would be easy enough implement and probably sensible with the octals. I always elevate the heater string to DC via the center tap, however.

ElectroVoice_TL606.pdf

If you have a 0-4-8-16 Ohm secondary, just connect the 4 Ohm tap to ground, and the 0 and 16 Ohm taps to the cathodes of the output tubes. You will get some cathode feedback together with the screen ultralinear feedback.

That's an interesting idea, Roberto. Yes, I do have 0-4-8-16R taps on the OT.

[roberto]

Just pay attention to be sure that the 0-4 and 4-16 sides of the secondary winding are quite similar in resistance, not to unbalance the output tubes.

[Colossal]

Just pay attention to be sure that the 0-4 and 4-16 sides of the secondary winding are quite similar in resistance, not to unbalance the output tubes.

Good tip, thank you. I will take measurements.

[Colossal]

Quick update:

1) TL606 cabs, Eminence CB15-8 bass drivers, and head cab are ordered
2) Chassis and metal valences are getting fabricated
3) Instead of using octal preamp tubes, I decided on 12AX7 (V1ab) and 12AT7 (V2ab and PI). Preamp design will be similar to a Plexi with modified values
4) Got my buddy a Tech 21 Geddy Lee YYZ pedal for Christmas. Killer pedal!

[roberto]

Hi Dave,

great you are going on with the bass amp, keep us updated!

[Colossal]

Here is the amp.

Four Svetlana =C= 6550s (thanks Xtian!) running in Class AB2 Ultralinear with Martin's 12AU7 driver circuit. Preamp is close to the TAD Plexi-ish bass preamp with a CF driven (modded) tone stack. Dumbley PI and no negative feedback. Plate voltage is running 457V, screens 455V, and 48mA at 120VAC mains. It got tested full force at band practice and works as advertised. It sounds very, very punchy. There is loads of bass on tap but it doesn't ever become mushy or incoherent. The EQ works well and there is a wide variety of sounds available. The design goal was to be able to get that Geddy Lee Moving Pictures tone and I think it really does capture that vibe well. My friend plays a maple neck Jazz Bass and with it, the sound is wonderfully warm, clear, and piano-like. He is very happy. Cabs are Thiele TL606 loaded with 15" 8Ω Eminence CB15 drivers. The transformers are very large and heavy but deliver the power. The whole power supply sees a 100uF - 10H/31Ω choke - 100uF CLC filter. The filaments are DC elevated via a hum-balance pot to keep the CFs happy. The amp is very quiet at idle.

V1 12AX7
V2 12AT7
V3 12AT7
V4 12AU7
V5 - V8 6550

bassamp.jpg

bassrig.png

frontpanel.png

A side note on this...I ran into a mishap on startup and wanted to mention it because it was quite unexpected, but also for the benefit of other builders. This stuff can catch you unaware sometimes, if you are not paying attention. Tiny things can slip through. I am usually very methodical and verify my work along the way of a build, testing continuity on boards, etc. When I loaded the power amp board in, I did not encounter any resistance, however, the screen supply lead serving the left pair got just ever-so-slightly pinched in the standoff! Unfortunately, I did not catch this until after startup. With the tubes unloaded, I brought the amp up on a variac to a low AC voltage just to check initial voltages. I noticed I had no voltage on the left side! Click. Pilot light goes off. Mains fuse pops. Oh, that can't be good

Initially, I thought I might have miswired the OT, not considering I had a dead short under the board. The OT leads were disconnected and it was then that I discovered the short after continuity testing. I found and removed the offending wire to find that the insulation had been flattened, leaving just a film of plastic on the lead wire. This was the root cause of the problem, for sure. I replaced the lead, reconnected the OT, powered back up slowly and all was as it should be. Loaded in tubes, biased right up, and left to idle for a bit. Test the amp and it sounded great! Huge thump with that Class AB driver. Came in later to "the smell" of wire insulation going down. Power down the amp! The PT laminations are very hot and the PT is done for, clearly approaching failure So, this little mistake/oversight ended up costing me a PT. First time I've killed one. Wish it could have been a Tweed Deluxe PT and not this behemoth, but it could have been worse. I was, very happily, able to secure a replacement (thank you very much Phil at Heyboer!) and the amp is up and running. It got thoroughly tested during my friend's practice this last weekend and it sounded really good (I've been sitting in with them this summer). The band is just simply way too loud ( ), but the amp cut through. Those big Thiele cabs really punch. The sound is very musical with that smooth, springy elastic quality you hear on recordings like Camera Eye and Red Barchetta. When pushed with a Tech 21 YYZ pedal, it gets a snarling, steely, shimmering wet distortion that sounds really good and very much like its namesake, YYZ.

My friend is really happy. He has already said he can tell he needs to improve his technique because the amp is very revealing. I built a 102 for the guitar player in the band as well. It's been a fun summer and I was glad to catch up with old friends and make some new ones.