TAB V76 Clone - Another Take

[Falk]

Good evening gentlemen,

about a year ago a had a job in Berlin and for a week I was commuting every morning and evening for a bit more than 30 minutes. I cant recall why I started to draw the V76 schematic in spice but a some point I had a well working model of the preamp. I played around with that model to get to know the circuit better. Also I read through the V76 threads here in GDIY and gathered quite a bit knowledge about the preamp. During the past year I could gather two Sowter V76 input transformers and four Lundahl anode chokes on Ebay. Output will be some Edcor iron I have here. From the U23 build I have power transformers and a PSU PCB what could do the job.

So I plan to build a stereo unit with PSU included. PSU is regulated with DC filaments. I needed to extend the PSU and designed a voltage quadrupler 12 -> 48V. I am quite curious if this will work. I designed pcbs along the tips and tricks I found in the other V76 threads. I integrated a DI input, skipped all the filter stages. Added in and output pad.

I will use EF86 or PF86 instead of the EF804. All capacitors in the signal path are film. Front panel comes from Frank. He did an awesome job again. I will drive the VUs from a chines VU buffer. The PCBs offers two positions to hook it up. I ll see what works best.

Today I assembled the front panel. Thanks again @gyraf for the knobs. I still live off of them. I will stuff the PSU PCB the next days and make a documentation of the build here. I hope you people enjoy the thread.

Best,
Falk

 

[Falk]

Yesterday I placed most of the components on the PSU PCB - I forgot to order one 22u cap.
I also prepared the PSU cables. I use an old PSU transformer which delievers 225VAC 120mA, 12VAC 1A, 6.3VAC 4A. The 225VAC will deliver 315VDC to the regulators. 12VAC will drive the 48V phantom board and also light up the LED switches and relays. 6.3VAC remains unused until I find something to use it for. The hammond delivers 12.6VAC for the filament DC. I designed the backplate so the hammond can be turned for minimal hum. The toroidal is mu metal shielded.

Today the other pcbs will arrive and I will go one placing components and finish the PSU.

Thanks for your attention,
Falk

 

[Falk]

Today the missing cap arrived and I installed the PSU with 48V extension. Everything works fine without load. 280V VB1, 240V VB2, 48V Phantom, 16V DC for LEDs & Relays, 12VAC for the VU buffer and driver.

I also mounted the chokes and input TXs. The output TXs will hang above the PCBS and are not mounted yet. In the next days I will place components on the signal PCBs and prepare the whole cables.

Thanks for chiming in!
Falk

 

[Falk]

So here comes todays update. I populated both of the signal boards and connected the iron to the circuit. Doesnt sound much but took me quite some time. Looking forward to start to build switches and connecting everything. Stupid me ordered the wrong relays so I needed to short the connections on the back of the pcb until new relays arrive.

More tomorrow! Thanks.

 

[Falk]

Today I did the front panel cables and switches. I am getting pretty close to fire the machine up but it will take at least one more day to have enough time to double check everything. I am getting nervous.

 

[Falk]

I finished the build and everything works which is great but there is a big BUT. There is way too much distortion and I have no clue where it arises from. Tonight or tomorrow I ll try to chase the signal with a scope to get a clue. The voltages are perfect in every position. May it be the 10k:600 Edcor output? The original transformer has 50k:600. May the search begin. It could also be the Lundahl chokes but they should have more than enough inductance.
I am very thankful for every thought on this.
Falk

 

[gyraf]

Does distortion clear up significantly at lower output levels? then I'd suspect the output transformer is not up for it

Otoh, if distortion seems somewhat ignorant to out levels, it could be a weak tube?

Are all voltages (=currents) within specs otherwise?

/Jakob E.

 

[Falk]

Hello Jakob (@gyraf), thanks, again! The voltages are all completely along the specs. Like within 5% or so. I scope through the signal path as soon as possible and also I ll feed audio to different stages. I am not assuming bad tubes as they are all NOS and both channels sound exactly bad in the same way. So it must be something structural.

 

[Falk]

Okay, I just bypassed the 600 ohms output pad and voilla - it works. I tried Manfreds T188 Haufe clones for the output and they sound even more like old broadcast than the Edcors. The sounds much better now.

Has anyone a hint how to realize an output pad after the output tx?

Thanks,
Falk

 

[ruffrecords]

Can you post the schematic of the output pad you bypassed?

Edit: One test I would do is load the output with 620 ohm resistor and see if the distortion returns.

Cheers

Ian

 

[Falk]

Thank you Ian @ruffrecords - It was a pad I found in a schematic by Dave Pendrill and successfully used in an altec clone.

 

[ElSmurf]

Congrats on the beautiful build. Those pcbs look great. Any chance you will put these up for sale at some point?

 

[ruffrecords]

Thank you Ian @ruffrecords - It was a pad I found in a schematic by Dave Pendrill and successfully used in an altec clone.

OK, that is a very straightforward design and should work. To the amp it will look like a 600 ohm load most of the time so I would expect you will get the distortion back if you load the output with a 620 ohm resistor. If you do then let me know.

Cheers

Ian

 

[gyraf]

If the amplifier tends to sounds better at slightly higher load impedances, simply use e.g. two 1K in series with signals then 470R across - should give you -15dB and ca. 300 Ohms source impedance..

 

[Falk]

OK, that is a very straightforward design and should work. To the amp it will look like a 600 ohm load most of the time so I would expect you will get the distortion back if you load the output with a 620 ohm resistor. If you do then let me know.

Cheers

Ian

Thank you Ian! Yes indeed the distortion came back when I put 600 ohms across the output secondary winding.

 

[Falk]

If the amplifier tends to sounds better at slightly higher load impedances, simply use e.g. two 1K in series with signals then 470R across - should give you -15dB and ca. 300 Ohms source impedance..

Thanks Jakob! There is still a bit of this distortion audible but its kind of a sound now and not too much. I will anyway try your hint. You mean to put this resitive network on the secondary or primary winding? I am pretty sure if this wont do the trick, a bigger output tx will help. Was quite the same on the U23.

 

[Falk]

Congrats on the beautiful build. Those pcbs look great. Any chance you will put these up for sale at some point?

Thanks! To make some signalboards available and a bom isnt a biggie. I wont share the PSU as this is quite on the experimental side and I dont want to do a revision soon. But just to be said. Each channel is around 1k€. This isnt a cheap build.

 

[ruffrecords]

Thank you Ian! Yes indeed the distortion came back when I put 600 ohms across the output secondary winding.

OK, then the issue is with the amplifier. Could be one of several things. First you need to measure the output level at which distortion starts.

Cheers

Ian

 

[beatnik]

Could be possible output transformer has the wrong ratio ? The original should be something like 50K/600

UTM makes a cost-effective 76K/600 opt for the V72 which perhaps is a better fit than the 10K/600 Edcor

https://utmindustry.com/utm3575/

 

[Falk]

Thanks Ian and Beatnik, the more I play around with the amp the more I think the Edcor wont do the job. Most of the remaining distortion happens in the 2nd stage of the preamp. I fed audio to the second stage and its only a little less distortion. So yes, I ll try a different output tx asap.
And thanks for the hint with the UTM tx. I ll order it.