Which Ampeg B15 ... to build?

[JapanAxe]

I have been severely bitten by the valve amp-building bug during lockdown(s) and among the projects in contemplation is something closely based on an Ampeg B15. I have the schematics of the various versions, and a layout for the single-channel Ceriatone Aunt Peg, which provides the options of cathode bias (1964) or grid bias (1966). I am in the process of converting the layout into a schematic.

The versions I have been looking at are:

• B15N (1962) - cathode biased;
• B15NC (1964) - almost identical to the 1962 (different rectifier valve and screen resistors);
• B15N (1968) - grid biased, tone stack moved to after second gain stage; and
• B15NF (1968) - as B15N but with ultra-Hi and Ultra-Lo switches

I will be building this as a single-channel single-input head. It will be run through a Barefaced One 10, which sounded gorgeous with the PF50T that I used to own. I think I'm right in saying that the cathode biased and grid biased versions put out 25W and 30W respectively. I have yet to decide whether I want to include Hi/Lo switches or different bias options.

Are there any B15 connoisseurs out there that can fill me in on the pros and cons of the various versions?

[BassmanPaul]

Just for interest, how many tube amplifiers have you successfully built? 

Do you have the tools and test equipment necessary to ensure proper operation of the completed build?

There is a lot of stuff that you need to be aware of for your safety.

I'm not trying to put you off necessarily but these things do run on LETHAL voltages.

Edited December 27, 2020 by BassmanPaul

[JapanAxe]

16 minutes ago, BassmanPaul said:

Just for interest, how many tube amplifiers have you successfully built? 

Do you have the tools and test equipment necessary to ensure proper operation of the completed build?

There is a lot of stuff that you need to be aware of for your safety.

I'm not trying to put you off necessarily but these things do run on LETHAL voltages.

It’s a valid question @BassmanPaul

I come from a science background (physics) and have been doing electronics as a hobby for over 40 years. I got into valve amp builds with a 5E3 kit 4 or 5 years ago. During lockdown I rebuilt a previous 5E3 Micro build and scratch-built clones of the Trainwreck Rocket amp and Fender 6G15 outboard reverb.

I have a good quality Megger AVO, a variac with accurate voltage and current read-outs, and of course a dim bulb tester. I don’t have an oscilloscope but haven’t found that to be a drawback thus far.

I hope that sets your mind at rest!

In other news I have discovered that the Ceriatone Aunt Peg appears to be a cheeky copy of Ampeg’s Heritage B15.

[BassmanPaul]

IMO a scope and a signal generator are vital tools. They are how you can see if your amplifier has any spurious artifacts hiding where you can't see them.

I haven't used an AVO since the Sixties but they always give me the warm fuzzies when I think of them. These days Fluke digital meters have replaced them.

You seem to be aware of the pitfalls so I'll wish you luck with the build. What are you doing about a chassis?

Paul

 

[Hellzero]

I've been into tube design and repairs for decades. The scope is not a vital tool to my own opinion, but a shorting big wire made with two clip-on pliers (one red and one black for polarity plugging), a 5 Watts resistor and a LED is a real vital tool.

You know that tension may remain in the capacitors if not properly grounded when shut down, that's why shorting them before doing anything is really vital. And for those not knowing this, there's a negative tension in most tube amps (won't go into details) that you have to discharge from the capacitor(s) by putting the shorting wire (with a LED and resistor) the other way.

Another vital device, but for the amp this time, is a loading resistor. 😉 

That said, I owned the first reiusse made in the 90's bought new at the time (a hard to swallow pill) and it was a terrific combo.

I would go for the B15NF version with, as you wrote, the two biasing possibilities and also a solid state rectifier possibility.

It's a very nice project !

[JapanAxe]

9 hours ago, BassmanPaul said:

IMO a scope and a signal generator are vital tools. They are how you can see if your amplifier has any spurious artifacts hiding where you can't see them.

I haven't used an AVO since the Sixties but they always give me the warm fuzzies when I think of them. These days Fluke digital meters have replaced them.

You seem to be aware of the pitfalls so I'll wish you luck with the build. What are you doing about a chassis?

Paul

 

My AVO is in fact digital! I also have an old moving coil multimeter which is handy at times. I’ll probably pick up a ‘scope at some point and will then realise what I’ve been missing...

I intend to build a wooden head sleeve to make a good size match to my One 10. In the bottom 1/3 of that will sit a U- or C-section aluminium chassis which can be ordered to size from Modulus Amps. The final form factor will be similar to a Demeter head:

[Chimike]

Gotta subscribe to this !

I also caught the bug during lockdown, and my B15 build is coming along nicely.  I went for the 64/66 option, and found a build manual on the Trinity amps website that was very helpful. Another source of info was CtG B15.

Great project, go for it !

[JapanAxe]

11 minutes ago, Chimike said:

Gotta subscribe to this !

I also caught the bug during lockdown, and my B15 build is coming along nicely.  I went for the 64/66 option, and found a build manual on the Trinity amps website that was very helpful. Another source of info was CtG B15.

Great project, go for it !

Hang on Mike, this is not going to be a build thread! I'm just hitting on the BC hive mind for thoughts on the different B15 versions, ideally with some idea of what the sonic differences are e.g. 'feel' of grid bias vs cathode bias.

I already found the CtG page and downloaded the build docs. Thanks for the tip about Trinity Amps - just downloaded their stuff too. This is now giving me even more options, whereas I want to settle on a simple 1-channel build with Volume, Treble, Bass (and maybe a bright switch).

So @Chimike where's your build thread!?

1 hour ago, Hellzero said:

I've been into tube design and repairs for decades. The scope is not a vital tool to my own opinion, but a shorting big wire made with two clip-on pliers (one red and one black for polarity plugging), a 5 Watts resistor and a LED is a real vital tool.

You know that tension may remain in the capacitors if not properly grounded when shut down, that's why shorting them before doing anything is really vital. And for those not knowing this, there's a negative tension in most tube amps (won't go into details) that you have to discharge from the capacitor(s) by putting the shorting wire (with a LED and resistor) the other way.

Another vital device, but for the amp this time, is a loading resistor. 😉 

That said, I owned the first reiusse made in the 90's bought new at the time (a hard to swallow pill) and it was a terrific combo.

I would go for the B15NF version with, as you wrote, the two biasing possibilities and also a solid state rectifier possibility.

It's a very nice project !

Yes I have a capacitor discharge probe and a dummy load (both DIY of course).

[Chimike]

Ha, 

Well , it might be worth looking at the later SB12 circuit, using 6L6's, as that appears to be what you are describing . I often use an SB 12, and they are fabulous sounding, and lighter than my old B15 !

[BassmanPaul]

On 28/12/2020 at 06:09, Chimike said:

Ha, 

Well , it might be worth looking at the later SB12 circuit, using 6L6's, as that appears to be what you are describing . I often use an SB 12, and they are fabulous sounding, and lighter than my old B15 !

Just being picky but accuracy in this line of business is important:  The tube needed is a 6L6GC. The 6L6 is an earlier variant usually metal cased, think WWII, and completely unsuitable for a bass amp.

[JapanAxe]

5 minutes ago, BassmanPaul said:

Just being picky but accuracy in this line of business is important:  The tube needed is a 6L6GC. The 6L6 is an earlier variant usually metal cased, think WWII, and completely unsuitable for a bass amp.

Quite right, like many people I just say 6L6 when I mean 6L6GC.

[Beer of the Bass]

Outside of salvaging parts from old equipment, you'd be hard pressed to find a non-GC 6L6 today.

[Chimike]

Anyway, I blame my spell-checker !😀

Edited December 31, 2020 by Chimike

[Hellzero]

10 hours ago, Beer of the Bass said:

Outside of salvaging parts from old equipment, you'd be hard pressed to find a non-GC 6L6 today.

Indeed, all modern 6L6 have a Glass Container. 😉

[JapanAxe]

12 hours ago, Beer of the Bass said:

Outside of salvaging parts from old equipment, you'd be hard pressed to find a non-GC 6L6 today.

 

1 hour ago, Hellzero said:

Indeed, all modern 6L6 have a Glass Container. 😉

Yes, the only place I see the old metal ones is on restorations by Uncle Doug on YouTube!

[JapanAxe]

On 28/12/2020 at 11:09, Chimike said:

Ha, 

Well , it might be worth looking at the later SB12 circuit, using 6L6's, as that appears to be what you are describing . I often use an SB 12, and they are fabulous sounding, and lighter than my old B15 !

I looked at the SB12 schematic (see below). The preamp uses 12AX7s but I fancy doing a build with octal preamp valves.

Just out of interest, is there a name for that phase inverter arrangement?

[Beer of the Bass]

1 hour ago, JapanAxe said:

I looked at the SB12 schematic (see below). The preamp uses 12AX7s but I fancy doing a build with octal preamp valves.

Just out of interest, is there a name for that phase inverter arrangement?

It's a Cathodyne phase inverter. You see them in the smaller class AB Fender amps too, like the Princeton Reverb.

[Hellzero]

And it's a no gain phase inverter too  unlike the two other types of phase inverters : the long-tailed pair inverter and the paraphase inverter. They all have advantages and disadvantages, but the cathodyne one is certainly the most accurate phase inverter as you don't need to have totally paired triodes to work perfectly.

[Hellzero]

@JapanAxe : the above mentioned Uncle Doug has a tutorial about phase inverters IIRC. 😉

[Hellzero]

Oops, getting old, I forgot the SRPP phase inverter...

[JapanAxe]

Cheers chaps. My last PP amp build was the Trainwreck Rocket,  which is a stripped-down AC30 top boost and uses a long tailed pair PI.

[JapanAxe]

Well I found this video comparing a 1963 B15N and a 1971 B15S. They both sound great but they are running through different cabs:

 

[Chimike]

re the SB12 circuit.

Ah, but there was a later version , using 6l6gc's, however, it doesn't use octal preamp  tubes, either.

Neither does the B15S in that comparison

 

Edited December 31, 2020 by Chimike

[BassmanPaul]

On 31/12/2020 at 04:05, Hellzero said:

Indeed, all modern 6L6 have a Glass Container. 😉

The 'G' refers to a glass container. The letter following defines the variant. For instance I have, somewhere, a pair of 6L6WGB which is an earlier industrial version and not as capable as the GC. I tried those in a Marshall JTM45 and they worked well. The 5881 and 7027 are similar tubes.

When I got to North America I was actually surprised by how many different tubes could be amalgamated under the 6L6 banner. It's why it's important to state the full description for American tubes.

Edited January 1, 2021 by BassmanPaul

[agedhorse]

On 28/12/2020 at 01:35, Hellzero said:

I've been into tube design and repairs for decades. The scope is not a vital tool to my own opinion, but a shorting big wire made with two clip-on pliers (one red and one black for polarity plugging), a 5 Watts resistor and a LED is a real vital tool.

You know that tension may remain in the capacitors if not properly grounded when shut down, that's why shorting them before doing anything is really vital. And for those not knowing this, there's a negative tension in most tube amps (won't go into details) that you have to discharge from the capacitor(s) by putting the shorting wire (with a LED and resistor) the other way.

Never directly short a capacitor to discharge it. The currents can be high enough to damage the electrode lead-out bonds inside the cap. It's probably the quickest way to ruin a good high value cap. A 10-25 watt resistor with about 10 ohms is a good value, and you don't have to worry about residual voltages either.

Without a scope, you would need a mighty fine crystal ball to see what's happening in an amp. I haven't seen any effective crystal balls marketed towards the electronics service industry myself.