B15 type amp and double baffle speaker cab build

[beans-on-toast]

Thank you for the feedback! I have attached my notes which include the heritage b15 (HB15) gut shot and the supposedly belonging schematic, the reference schematic i used (and where i cut away channel two + minor mods) and my diy-wiring diagram (be aware: the diy-wiring diagram is a mess ). I went back and forth between the gut shot, the HB15 and the 196x schematic and made the wiring diagram. Further you can find the schematic i`ve drawn with some annotations. I hope, i`ve got it right Ah btw, both the wiring diagram and the schematic are more or less just a guide line when i`m working on the amp. On the one hand, I connected some turrets slightly different as shown on the diy wiring diagram. On the other hand, i will omitt some stuff (e.g. some fuses), which is shown in my schematic.

Your work is well thought out. Very nice drawing.

Some general comments:

The heritage schematic that you posted is the corrected one. The earliest one had an error in the fixed bias circuit as well as heater label errors.

The heritage ground scheme is not the same as the vintage eyelet board B-15’s. On the B-15, the ground bus flows from the power supply node A down to the input jacks. The ground point is at or near the input jack. If you use a cap can, isolate the can from the chassis. There should one be one ground to the chassis. It can be at the input jack or the other end of the ground bus, but not at both ends. Doing so would result in a ground loop.

I like the addition of the two diodes on the tube rectifier.

I like to ground the speaker return at the phase inverter ground point. This makes a quieter amp.

On the B-15, the main speaker out is isolated from the chassis, as is the ext spkr jack.

You are using a shielded wire from the input jack to the tube input. Only one end should be grounded. With a ground on the isolated jack, there isn't a ground on the cable.

I prefer to use shielded wire to the tone controls rather than twisted wires.

For best noise immunity, the 120K grid stopper resistor is best placed at the tube socket of V1. Keep the leads as short as possible, anything that extends beyond the shield can pick up noise. Ampeg didn't do this, they should have.

Your power transformer has a CT heater winding. Use that if you don't want to use a heater hum balance pot. Use one or the other but not both.

 

[dr_root]

Thank you for the feedback! I have attached my notes which include the heritage b15 (HB15) gut shot and the supposedly belonging schematic, the reference schematic i used (and where i cut away channel two + minor mods) and my diy-wiring diagram (be aware: the diy-wiring diagram is a mess ). I went back and forth between the gut shot, the HB15 and the 196x schematic and made the wiring diagram. Further you can find the schematic i`ve drawn with some annotations. I hope, i`ve got it right Ah btw, both the wiring diagram and the schematic are more or less just a guide line when i`m working on the amp. On the one hand, I connected some turrets slightly different as shown on the diy wiring diagram. On the other hand, i will omitt some stuff (e.g. some fuses), which is shown in my schematic. Wiring this amp is not a 100% strict undertaking.



Hmm...it was not very expensive but its fixed with super glue:/ it wouldnt be so easy to remove it without destroying the grill cloth. Lets see..maybe i will change it though.

i just realized, that i uploaded an outdated version of my schematic and i cant edit the post for some reason. attached you find the latest schematic.

 

[beans-on-toast]

i just realized, that i uploaded an outdated version of my schematic and i cant edit the post for some reason. attached you find the latest schematic.

Always connect the power cord ground wire to separate ground with no other connections to that point. Electrical codes require this. There should be a ring terminal and lock washer and nut on the fastener.

Some use a power transformer leg if they don’t want a separate chassis hole drilled for this mounting point.

Multi star grounds can work if done properly.

 

[Feral Feline]

@dr_root
Thank you very much for sharing your notes.

@beans-on-toast
Many thanks for sharing your knowledge, I can use some of it in my Little Princeton build.

Had hoped to finish the Princeton before the holidays, but a schedule change prevented that. Now hope to finish it before 2021... which is dependent on my Amp-Sifu's schedule...

I think I better heed his advice and build another small amp (probably a few) before tackling the B-15.

 

[beans-on-toast]

@dr_root
Thank you very much for sharing your notes.

@beans-on-toast
Many thanks for sharing your knowledge, I can use some of it in my Little Princeton build.

Had hoped to finish the Princeton before the holidays, but a schedule change prevented that. Now hope to finish it before 2021... which is dependent on my Amp-Sifu's schedule...

I think I better heed his advice and build another small amp (probably a few) before tackling the B-15.

I’m doing a Deluxe cab as we speak. Ask away when you get to it.

 

[Feral Feline]

 

[dr_root]

Hi guys,
During the last two weekends I found some time to work on the wiring of the amp. Apart from the DI-components and the power indicator led, everything of the amp itself should be wired. It's a bit more messy than I was hoping for, so let's see how it will perform. Right after drilling the mounting holes in the chassis, I realized that I messed up mounting orientation of one the transformer..I planned to mount one of the two 90° rotated with respect to the other...mhmmh but now I don't want to redo it...I hope it performs well.
I didn't dare to power it up without a Variac, which is in the mail and should arrive soon.
I made some small changes/mods compared to the B15 which made sense for me:

-Since I followed a star ground scheme, I added a 15nF cap right at the input from the preamp GND (PAGND) to the earth GND
-i changed the input voltage divider, comprised of the grid stopper resistor and the 5.6M pull down. The pulldown resistor now directly connects the input signal and PAGND, while the grid stopper resistor is now located right at the tubes grid pin. This should not affect the input signal in a practical manner but improve the rf interference situation
-since the standby switch is not needed according to some sources (like valve wizard) if you use a tube rectifier, I use the switch to be able to mute the power amp. If the switch is closed it shorts the volume pot wiper to the phase inverter GND, such that the subsequent stages have GND level input signals, which should result in a muted output
- I installed two 150 Ohm 25W power resistors in series to the output connector. This way, the amp can be operated without any external load. The resistors are bolted to the aluminium chassis and should be able to handle the thermal load,even if the amp is fully cranked.

The only thing missing so far are two DI transformers. I ordered them directly at sowter by the end of October and the shipping was planned by the beginning of December..but here we go..I don't have any information when they arrive.
The DI PCB boards are done already and next time I will have some spare time, I will build the DC power supply for the DI circuitry.

cheers
root

 

[dr_root]

Hey guys, im a bit freaking out at the moment. Right now I am completely unsure whether the chosen power transformer is ok or if it's giving a bit to much output voltage. I think I'm missing something and hence I'm on the wrong track. I can't comprehend how the guidance DC voltage readings in some b15 schematics are obtained. If for example you have a 750 VCT power transformer like the pt108 or the Hammond 374bx which I use, the DC voltage at the first part supply filter cap usually is stated to be approx. 440V.... But how? Let's assume you have 375VAC, peak to peak that's around 530V. If we rectify that and take into account around 50V voltage drop of a 5u4gb rectifier tube, that's still 480VDC. Where are the remaining 40V being dropped to receive the stated 440VDC?

Thanks for your help!
root

 

[beans-on-toast]

It isn’t as straight forward as one would think.

Transformer voltage and currents are RMS unless otherwise specified. Specs are under rated load. The voltage will be attained at maximum current. If you aren’t drawing maximum current, the voltage can be higher.

The 5U4GB voltage drop relates to current conduction. At 275mA through each plate, the drop will be 50V. At a lower current, the voltage will be lower.

Transforms are not 100% efficient. There are load losses, core losses, flux leakage. Energy is dissipated as heat. Operating specifications take this into account. Everything depends on how hard the transformer is pushed.

 

[dr_root]

Thanks for your feedback.. The only thing I did not consider was the efficiency. The voltage under load seems to be the problem in my case. Since I'm building a slimmed down b15 version, the load will be less than in the original version. When I bought the currently installed transformer, I was not sure what I will eventually build, so I was just looking for stuff that was used as replacement parts for full B15/B15 Clone builds... Don't do that finish the BOM before you build something...

To not fry the first filter cap and to lower the voltage in total, I just ordered a different power transformer (Hammond 372bx), which outputs 50Vrms less than the 374bx. So I hope by the end of the week it will be installed and I will be able to finally power it up the first time.

 

[beans-on-toast]

Thanks for your feedback.. The only thing I did not consider was the efficiency. The voltage under load seems to be the problem in my case. Since I'm building a slimmed down b15 version, the load will be less than in the original version. When I bought the currently installed transformer, I was not sure what I will eventually build, so I was just looking for stuff that was used as replacement parts for full B15/B15 Clone builds... Don't do that finish the BOM before you build something...

To not fry the first filter cap and to lower the voltage in total, I just ordered a different power transformer (Hammond 372bx), which outputs 50Vrms less than the 374bx. So I hope by the end of the week it will be installed and I will be able to finally power it up the first time.

With the 750VCT it is what the original spec called for. That voltage is one of the secrets of the B-15’s power stage. I assuming that you are wiring for 240VAC and not for 230V. A bit higher is not a problem. If it’s way off, you can add a power zener diode and connect it between the high voltage centre tap and ground. The problem is heat dissipation, you’d need to have chassis space to mount it.

It is good to select a high voltage capacity filter cap in the first position. That one is stressed more than the downstream ones when charging when you first turn the amp on.

 

[beans-on-toast]

I should have added, connecting capacitors in series allows you to sum the voltage capacity, so for example, two 400V capacitors will yield an 800V capacity. Good for node A. I use a small PCB with snap-in caps for this.

 

[dr_root]

The two resistors which can be seen are there to balance the voltage between the two caps,right? What are their values?
500k range?!
That would probably be both the easiest and cheapest option, respectively

 

[beans-on-toast]

The two resistors which can be seen are there to balance the voltage between the two caps,right? What are their values?
500k range?! That would probably be both the easiest and cheapest option, respectively

Yes, they are balancing resistors. They also serve as a pathway to ground when the power is turned off. A good thing.

The attached file shows you how to calculate the value. I typcally use 220K 2W.

The bare PCB is $17.95US delivered anywhere in the world for three boards. It's available here: OSH Park ~, I have no financial interest in this, just making it available. I'm sure it would be less if made in China.

 

[MYLOWFREQ]

WOW!

 

[Feral Feline]

Beans-on-Toast, many thanks for sharing the BR PDF and your OshPark daughterboard.


I got some more work done on the Little Princeton at last, posted about it in the DIY pedal thread here.

Fired up, so beginning to feel like I may be able to tackle the B-15 down the road — definitely after a few more smaller builds.

 

[dr_root]

Hi everyone, I finally found some time to continue working on the amp. I added a first filter cap daughter board as suggested by beansontoast..its working as expected.

Today I fired it up the first time...in the beginning I managed to mess up the tube arranement and put the 6sl7s in the 6l6 positions and vice versa...dang..anyways at some point I realized my fault when checking voltages. After I swapped the tubes I had a horrible squeezing + a kind of popcorn noise. The squeeling sound went away when I disconnected the feedback resistor from pin6 of the upper phase inverter t (schematic vise). The popcorn noise went away mostly when I swapped one 6l6 with a fresh one. it is now mostly gone but not completely..it is sometimes there and sometimes not.. sometimes you can here it when the mute switch is on and sometimes not...I guess I messed up the tubes..I'll have to order some fresh ones to find out more...I thought that my routing was too messed up so I rerouted some wires, shortened some wires but that basically didn't changed anything...so I'm hoping for the new tubes.
Further I am really confused about the feedback leading to.that squeeling sound. My first guess was that I connected it incorrectly to the output plug or to the wrong pin of the phase inverter tube. However, i double checked that and I am pretty confident that I got it right. I also tested what would happen if I connected it to pin3,but that just made it slightly more noisy. Do you guys have some ideas?

Apart from the two bugs it sounds amazing. Even though I am testing it with a small speaker cab not intended for the situation and the feedback being disconnected, it already gives an incredible sound, I am so pleased by every single note played....I am still so pumped...I can't wait to test it on the cabinet I built. I am now also postponing an anticipated pickup. Change because the sound is so nice...I am really stoked!



Cheers

 

[beans-on-toast]

Hi everyone, I finally found some time to continue working on the amp. I added a first filter cap daughter board as suggested by beansontoast..its working as expected.

Today I fired it up the first time...in the beginning I managed to mess up the tube arranement and put the 6sl7s in the 6l6 positions and vice versa...dang..anyways at some point I realized my fault when checking voltages. After I swapped the tubes I had a horrible squeezing + a kind of popcorn noise. The squeeling sound went away when I disconnected the feedback resistor from pin6 of the upper phase inverter t (schematic vise). The popcorn noise went away mostly when I swapped one 6l6 with a fresh one. it is now mostly gone but not completely..it is sometimes there and sometimes not.. sometimes you can here it when the mute switch is on and sometimes not...I guess I messed up the tubes..I'll have to order some fresh ones to find out more...I thought that my routing was too messed up so I rerouted some wires, shortened some wires but that basically didn't changed anything...so I'm hoping for the new tubes.
Further I am really confused about the feedback leading to.that squeeling sound. My first guess was that I connected it incorrectly to the output plug or to the wrong pin of the phase inverter tube. However, i double checked that and I am pretty confident that I got it right. Do you guys have some ideas?

Apart from the two bugs it sounds amazing. Even though I am testing it with a small speaker cab not intended for the situation, it already gives an incredible sound, I am so pleased by every single note played....I am still so pumped...I can't wait to test it on the cabinet I built. I am now also postponing an anticipated pickup. Change because the sound is so nice...I am really stoked!

Cheers

Mistakes happen.

Try swapping the two primary output transformer wires (the two that connect to the tube plates), reconnect the feedback resistor, test the amp.

 

[dr_root]

Thanks for your feedback...I'll test that tomorrow right after breakfast.
Man I'm still so amazed by the sound..it's so addictive

 

[Andy Marshman]

I got a B15 about 10 years ago (now sold). I stripped off the beat up tolex and refurbished the entire thing including the 'cart', replacing flip top latches too. I upgraded all the caps and restored the XLR speaker connector on the side of the cab to original working condition. I was able to contact Jess Olliver (designer of the amp for Ampeg) during the restoration and he was a super gentleman (RIP). I wanted to love it, but really needed something louder, despite knowing it was a low power amp. I took photos throughout the rebuild but those are on some backup hard drive somewhere around the house.
The baffle design, in my opinion, didn't do anything overwhelmingly special. Over-rated in my opinion. It's functional and 'different' but no big deal. I have a good ear for sound, but as I said, didn't catch anything special with the baffle design. I should add that after owning a variety of Ampegs, they are not 'my sound' at all. I'm not knocking Ampeg; just not for me.