Tube Town B-Flex

[XjFred]

Hello, everyone!

I’m planning to build a tube bass amp using this kit from Tube Town in Germany. It’s a kit inspired by Ampeg’s Portaflex series.





I’m thinking of making a few modifications:
- Add a volume potentiometer (1 Mohm log) in place of resistors R14 and R15. Passinwind recommended a useful improvement: using a switchable potentiometer to add a bright cap, but I’m having trouble determining the value of that capacitor ( fc=1/2πRC).
- Use output jack with switching contact to incorporate a dummy load and prevent damage if the amp is plugged in without a load.
- Add a DI output before the volume knob
- Add a Stand-By switch on HV1/HV2.

I’d love to hear your thoughts. I’ve already built a pair of pedals based on kits designed by Nathan from Sushibox FX. Succesfully and I'm having lot of joy using them. But this will be my first DIY amp. I plan to connect it to an Ampeg SVT 210 cabinet.

 

[Asdrael]

The standby switch isn’t really useful. A MV definitely is. You could also play with changing the tonestack structure to give you more control and possibly make the bright cap on the MV less important.

 

[XjFred]

Do you think I should put the MV before the tonestack ? Is that what you mean by "changing the tonestack structure" ?

 

[pbrommer]

1. If I did a master volume, I’d use a post-phase inverter master volume. The Lar-Mar example works well.
2. The dummy resistor is going to be larger in size, so allow space!
3. DI out might work best off the R14/R15 going into the phase inverter.

4. I would see if there’s a way to bring down the plate voltages of B+1 and B+2. Sure, some 6v6gt can take up to 400vdc, but you’ll have to watch the bias. If you can get to 375vdc, they’ll be a bit happier

 

[JohnnyCreepy]

Quick look on schematic had me thinking it might be a noisy circuit and need tweaks f.ex. input resistor values.

Use output jack with switching contact to incorporate a dummy load and prevent damage if the amp is plugged in without a load.

Just solder 470-1000ohm 5W resistor across output jack leads and call it done.

but you’ll have to watch the bias

With that supply; definately!

Easy to do mods with turret boards. But why I haven’t looked TT kit prices before? Really affordable kits!

 

[XjFred]

Thank you both. These are some things to think about before I place my order.

1. If I did a master volume, I’d use a post-phase inverter master volume. The Lar-Mar example works well.

I've found this schematic.



If I understand correctly, I'll need to replace R24 and R25 with a two-stage 250kohm log potentiometer. And add 2x1.8Mohm safety resistors to maintain the initial values of approx 270k (220k in that case).

I also found a variation (Frondelli MV) :



Any thoughts about this one vs the Lar-Mar solution ?

2. The dummy resistor is going to be larger in size, so allow space!

You're right. I won't have enough space in the chassis to afford 3 power resistors for the outputs.

4. I would see if there’s a way to bring down the plate voltages of B+1 and B+2. Sure, some 6v6gt can take up to 400vdc, but you’ll have to watch the bias. If you can get to 375vdc, they’ll be a bit happier

OK, but won't that reduce the amp's power ? It's only 18W...

Quick look on schematic had me thinking it might be a noisy circuit and need tweaks f.ex. input resistor values.

Noise is definitely the last thing I want. What do you recommend regarding the input resistors ? Are you talking about R1 and R2 ?

Just solder 470-1000ohm 5W resistor across output jack leads and call it done.

OK, jacks with contact switch and 1Mohm resistors (only 5W ??) and this will allow me to use the DI out without any cab plugged in ?

As for adjusting the bias, I'm going to purchase this tool for precise calibration :

Bias-Probe Octal, KIT

Kit of a bias probe for indirect measurement of the quiescent bias current of a tube. The adapter is placed between the tube and the socket and is connected to a standard voltmeter (not included). Pay attention to the quality of the meter and to ...

www.tubetown.net

Again, thank you both. A lot of food for thougts here.

 

[JohnnyCreepy]

OK, jacks with contact switch and 1Mohm resistors (only 5W ??) and this will allow me to use the DI out without any cab plugged in ?

You misread good, for clarification and hazard safety note 470r-1000r aka 470 ohm to 1 kohm.

Bias probe is a gimmick if you don’t bias as your profession. Just stick individual 10r resistors between tube socket pin1+8 and GND. That’s your bias probe there to take accurate enough DMM reading for biasing. Just make sure you match resistors to each other close enough.

Noise is definitely the last thing I want. What do you recommend regarding the input resistors ? Are you talking about R1 and R2 ?

It really depends what you’re using the amp with. Maybe use stock values first and if needed adjust from there, 20-33K/470k-1M? But will it then have PortaFlex sound anymore, dunno!

 

[pbrommer]

Type2 Lar-Mar is the one that a lot of DIY people use. Make sure to have those safety resistors (2.2M is pretty common)

Dropping the voltage a bit does drop some output, but the difference between 5W (like a champ) and 18W when fully cranked is different but not insanely different (because volume and decibels are logarithmic).

I should stop before I get too deep over my head in knowledge. I know enough, but I haven’t built an amp in a few years.

 

[Asdrael]

Do you think I should put the MV before the tonestack ? Is that what you mean by "changing the tonestack structure" ?

I think a MV between tonestack and PI is sufficient, but some prefer a post PI MV.

And by changing the tonestack I mean using something more classical like a Marshall or Fender TMB topology.

As for the bias probes, to be honest, if you are building an amp, you shouldn't be needing those... just a multimeter on a pin with an alligator on the chassis ground screw and roll really.

 

[XjFred]

Thanks for these advices.

And by changing the tonestack I mean using something more classical like a Marshall or Fender TMB topology.

I need to think about that—it might be interesting to have a mid-range control. But I'm afraid that if I give up the Baxandall tone stack, I'll lose the “Ampeg Portaflex sound" (even if SVTs are famous for their midrange control...).

 

[JohnnyCreepy]

@XjFred I recommend consuming these:

Valve Amps: Valve (Tube) Amps

Classic valve amplifiers

education.lenardaudio.com

Amplified Music : Guitar Valve Amp Service

How to service a Valve Tube guitar amp

education.lenardaudio.com

Also good reads on Valvewizard site, Aiken amplifications white papers-section and using search function to find valuable conversations from el34world, music-electronics forum and diyaudio.

Edit. Bass amp with 2x6v6 output tubes seems bit shy powerwise. Where do you plan using it?

 

[XjFred]

@XjFred I recommend consuming these:

Valve Amps: Valve (Tube) Amps

Classic valve amplifiers

education.lenardaudio.com

Amplified Music : Guitar Valve Amp Service

How to service a Valve Tube guitar amp

education.lenardaudio.com

Also good reads on Valvewizard site, Aiken amplifications white papers-section and using search function to find valuable conversations from el34world, music-electronics forum and diyaudio.

Edit. Bass amp with 2x6v6 output tubes seems bit shy powerwise. Where do you plan using it?

I plan to use it in my home studio or for small blues jam sessions with my guitarist friend.
I have an Ampeg Micro VR, a 160W solid-state amp, for when I need more volume.
The goal of this DIY project is to have a small tube amp at home, but above all to learn and expand my knowledge.
This is my first build. The next one will probably be more powerful (I’d like 100W, like the Ampeg V4) and will benefit from the experience I’ve gained with this first Tube Town amp.
That’s why I want to thank you for the reading recommendations you’ve given me. I’m going to dig into them carefully.

EDIT : Maybe these articles will teach me how to add an extra power stage and boost the power of this kit. But honestly, that's not my plan for this first kit.

 

[Asdrael]

As a first build just go with the schematic, make it work well and silently and then iterate/mod it imo. Get comfortable with it.

 

[GAStan]

Since it hasn't been mentioned yet, SAFETY! Be aware of what you’re working on at all times, these voltages can kill you and at the very least hurt like hell if you touch the wrong thing (speaking from experience).

On the bias probe, I agree with @JohnnyCreepy except I prefer to use 1r resistors because the math is easier: 1mV=1mA. I install these with an external test points on my amps so I can set/check bias without opening up the amp. Install one 1ohm resistor per power tube between cathode and ground. I use 1 watt resistors but have read where others prefer 1/2 watt. This reading will be slightly higher than actual anode current because both anode and grid current flow through cathode.

My preference is the pre-inverter volume control you mention in post #1. This is actually how Dumble did it in his amps that have been documented.

 

[XjFred]

Since it hasn't been mentioned yet, SAFETY! Be aware of what you’re working on at all times, these voltages can kill you and at the very least hurt like hell if you touch the wrong thing (speaking from experience).

As a chief engineer in the merchant marine, I hope being able to stay vigilant, although I also know one should never be too confident in this kind of situation. Thank you for reminding us of this basic principle: high voltage can indeed cause serious harm.

On the bias probe, I agree with @JohnnyCreepy except I prefer to use 1r resistors because the math is easier: 1mV=1mA. I install these with an external test points on my amps so I can set/check bias without opening up the amp. Install one 1ohm resistor per power tube between cathode and ground. I use 1 watt resistors but have read where others prefer 1/2 watt. This reading will be slightly higher than actual anode current because both anode and grid current flow through cathode.

That's a great idea; I'll look into where to place the bias adjustment on the back panel. I'll also need to put the adjustment potentiometer there.

My preference is the pre-inverter volume control you mention in post #1. This is actually how Dumble did it in his amps that have been documented.

I'll try to find the Dumble thread about it but can you explain why this is your preference vs Lar-Mar MV ?
At first, I opted for the MV after the tone stack, based on how my C2CE pedals are built.

 

[GAStan]

I'll try to find the Dumble thread about it but can you explain why this is your preference vs Lar-Mar MV ?

Here are several examples of Dumble amps with schematics in PDF format. Serial#s 102, 124, & 183 as well as the Steel String Singer (SSS) sticky threads would be good places to start.

Consequently I have built four different 100 watt amps based on #s 102, 124, 183 and SSS 002, the SSS is my avatar here. I am near completion of a Bluesmaster which will be my fifth 100 watt amp.

To be honest, it is my preference because it is all I have used thus far. It has worked well for me so I have not really considered anything else. If I were in your shoes I would likely use it because it alters the existing circuit very little and is easier to implement. For me to speak of the Lar-Mar would be unfair to it and you because I would just be repeating what I have read on the internet rather than speaking from personal experience.

I'll also need to put the adjustment potentiometer there.

This is the pot I use for bias.