[BUILD] NU VARIMU 436 build & help thread

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petermontg

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Dec 29, 2012
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191
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Ireland
There's a bunch of 3575's on eBay at the moment if anybody is going the sowter route on input.

I picked up a couple of pairs.

Best
Peter
 

HerbertR

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Mar 3, 2020
Messages
91
Location
Austria Voecklabruck
I have also finished my build. I ve used LL1540s the Sowter 1232 for my build. I could grab a pair of small 1mA Honeywell MM1 Meters on Ebay for about 65 Euro. I also tried to buy directly from Jewell Company NOS MM2 Meters. But their behavoir was so rude - neeeevvvveerr again. So a snooty, disgusting company. Bäaah...👎
The Compressor is in my opinion super clean, really fast, it has a very expensive sounding texture. Its difficult to get this piece really dirty sounding, hard to get it pumpin'. Like SSL on Tubes.... Wow....

Nothing to mention, no real problems with the build. It works excellent.
My Power TX is a customer made TX, with 0.2A 215V secondary, 4A 8V secondary and 1A 12V secondary from Audiophonics France. I have 315V DC after the B+ Bridge and about 15V Voltage Drop over R1. As for B+ Rtrim I ve used 1K -> total 48K for R3+Rtrim to get about 260V. That works super and the Components like the heatsink and resistors only gets slighly warm within the B+ PSU circuit. The heater PSU Circuit also works perfectly. My secondary shows 8.3V AC. The Input Voltage before of the TL338 regulator shows no necissitiy to tweak the Voltage down. The heatsink gets not really hot.
As for the meters I also think you will really need a 1mA DC Meter or they will not get adjusted within the range. The Meter adjustments also affect the other calibration adjustments. So it required more calibration cycles. I used a 1kHz Signal with +5dBu on the input and adjusted a 5dB Compression / Voltage Drop to 0dBu on the Output with the compressor. After that I adjusted the Meter Zero with the Multiturn Potentiometer and tried different Resistor in parallel to get the 5dB GR on the Scale and Zero in Bypass. It ended with 100 Ohms parallel between the Meter. Its a funny work. I also "slowed down" the meterspeed with a 10uF Capacitor parallel to the meter. Okay, you should not expect a high precission device. But it works pretty well. Only on high Compression Values bigger 15dB my two Channels show a difference of about 1.5 dB and more due my 6BZ7 Tubes are not really matched and should be replaced as last part. Changing of the 6BZ7 Tubes between the two channels clearly showed the difference.
Thanks to Bernbrue and Zyance - you are amazing!
 

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sbranco

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Jul 25, 2005
Messages
86
Location
Northwest NJ
I am a complete amateur but have managed to build a couple Seventh Circle N72's, a couple API 312's using rob higgins pcbs from years ago and four of mitsos John Hardy style M1 preamps.

I think I have enough preamps though never got around to doing a tube preamp.

Instead, I was thinking of building a tube compressor and this seems like a good candidate.

Bernd recommended using parts available and I was wondering if the transformers I have on hand would work or why they would not.

I have Jensen JE-8251 , 1to1 transformers which worked well as output transformers in the John Hardy M1 preamps. I also have Altran C-3402-2 1:8 mic input transformers also used in the John Hardy M1s.

Also, I got overzealous and got some tubes thinking of doing a tube preamp and never used them. I have 6AK5, 6072A, E80F, 6AU6 and 6AU6A tubes in my closet.

The transformers seem like they might work well. Would the Altran's fit the pcb?

Read through this thread a couple times and have gleaned a lot. You guys sure do some nice work.
This project would be a stretch but I feel a lot better working with the voltages having a pcb to help keep things straight. Test voltage spots and tips should help too.
 

Rob Flinn

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Joined
Jun 3, 2004
Messages
4,732
Location
Sussex, UK
The jensen would possibly work as an input transformer if it's 10k:10k & has a centre tapped secondary. The Altran is not right for this project either as an input or output transformer. If you look at the Sowter website or UTM industries websites where they sell transformers that are suitable. Or you could just read the information on this thread to give you an idea of the specs of the transformers & what people are using.
 

rockinrob86

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Joined
Mar 9, 2015
Messages
238
Location
Tampa, FL
I have also finished my build. I ve used LL1540s the Sowter 1232 for my build. I could grab a pair of small 1mA Honeywell MM1 Meters on Ebay for about 65 Euro. I also tried to buy directly from Jewell Company NOS MM2 Meters. But their behavoir was so rude - neeeevvvveerr again. So a snooty, disgusting company. Bäaah...👎
The Compressor is in my opinion super clean, really fast, it has a very expensive sounding texture. Its difficult to get this piece really dirty sounding, hard to get it pumpin'. Like SSL on Tubes.... Wow....

Nothing to mention, no real problems with the build. It works excellent.
My Power TX is a customer made TX, with 0.2A 215V secondary, 4A 8V secondary and 1A 12V secondary from Audiophonics France. I have 315V DC after the B+ Bridge and about 15V Voltage Drop over R1. As for B+ Rtrim I ve used 1K -> total 48K for R3+Rtrim to get about 260V. That works super and the Components like the heatsink and resistors only gets slighly warm within the B+ PSU circuit. The heater PSU Circuit also works perfectly. My secondary shows 8.3V AC. The Input Voltage before of the TL338 regulator shows no necissitiy to tweak the Voltage down. The heatsink gets not really hot.
As for the meters I also think you will really need a 1mA DC Meter or they will not get adjusted within the range. The Meter adjustments also affect the other calibration adjustments. So it required more calibration cycles. I used a 1kHz Signal with +5dBu on the input and adjusted a 5dB Compression / Voltage Drop to 0dBu on the Output with the compressor. After that I adjusted the Meter Zero with the Multiturn Potentiometer and tried different Resistor in parallel to get the 5dB GR on the Scale and Zero in Bypass. It ended with 100 Ohms parallel between the Meter. Its a funny work. I also "slowed down" the meterspeed with a 10uF Capacitor parallel to the meter. Okay, you should not expect a high precission device. But it works pretty well. Only on high Compression Values bigger 15dB my two Channels show a difference of about 1.5 dB and more due my 6BZ7 Tubes are not really matched and should be replaced as last part. Changing of the 6BZ7 Tubes between the two channels clearly showed the difference.
Thanks to Bernbrue and Zyance - you are amazing!
Your build looks great! Where did you get the facepanel? It is the only thing holding me back from building this (I hate metal work!)
 

HerbertR

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Joined
Mar 3, 2020
Messages
91
Location
Austria Voecklabruck
Your build looks great! Where did you get the facepanel? It is the only thing holding me back from building this (I hate metal work!)
Hi, Frank from frontpanels.de has done the frontpanel on green anoidizised Alu. If you need a .fpd file for the frontpanel designer software I can mail it to you. Totally easy free software to modify the holes, signations....Br
 

sbranco

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Joined
Jul 25, 2005
Messages
86
Location
Northwest NJ
I am gonna go slow but am trying to convince myself that I really need the iron before I order some.

For the input transformer, the pcb allows for an API2622, which is a mic input transformer. From what I can gather, the EA2622 and the Altran C3402-2 are pretty much equivalent. UTM seem nice but the pins don't match the pcb configuration. Am I missing something as to why the Altran would not work?
 

sbranco

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Joined
Jul 25, 2005
Messages
86
Location
Northwest NJ
Feel like I am being cheap trying to reuse parts but cheap is relative. Would many use this site if they could just go out and buy what they wanted? Well OK some guys just like to do this stuff regardless. And I am thankful for all the info. Wish this project had as much activity as the Sta-Level or the transformer-less varimu!

Anyway, my plan is to gut an Altec 1707c, a powered PA mixer/amp. To that end I would like to recycle/reuse what makes sense from it.

This is the EI power transformer. It is wired for 120V. For the secondary I am reading 50Vac for the blue wires. I get 80Vac between the larger red wires. 228Vac between the red wires after what I think is the yellow/black center tap? And 113Vac between each red wire and the yellow/black center tap? Haven't tested the smaller red wire because I'm not sure what to test it against. It is soldered to a post on the pcb marked "AC". This transformer has a copper flux band around it. It is a pretty large case so I think it could be far enough to keep anything audible from creeping in. 3.5 Amp fuse by the IEC inlet, 165 Watts maximum. I have read a lot about transformers but was wondering if anyone has repurposed these. The 50Vac is too hot but that can be reduced fairly easy no? Anyone know if I am right about the secondaries? The schematic is readily available for the 1707c but not much info on the power transformer.

001.jpg

Plan to try using the Alltran input transformers but have ordered some UTM output transformers designed for this.
 

Rob Flinn

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Joined
Jun 3, 2004
Messages
4,732
Location
Sussex, UK
I am gonna go slow but am trying to convince myself that I really need the iron before I order some.

For the input transformer, the pcb allows for an API2622, which is a mic input transformer. From what I can gather, the EA2622 and the Altran C3402-2 are pretty much equivalent. UTM seem nice but the pins don't match the pcb configuration. Am I missing something as to why the Altran would not work?
Not quite sure why they use a mic amp transformer. The original RS124 uses a 1:1. I haven't built this box or used these pcb's, my builds are more an exact copy of the RS124 circuit but my own point to point layout, but I definitely didn't need any extra gain using a mic transformer would have supplied.
 
Last edited:

sbranco

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Joined
Jul 25, 2005
Messages
86
Location
Northwest NJ
Hey Rob,
Just wondering if there was something I overlooked. You are far ahead of me with this stuff so thank you for your thoughts. I will probably need more help as I get further along. The answer is probably already on this forum - somewhere.
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,732
Location
Sussex, UK
Hey Rob,
Just wondering if there was something I overlooked. You are far ahead of me with this stuff so thank you for your thoughts. I will probably need more help as I get further along. The answer is probably already on this forum - somewhere.
I haven't built this particular project. It is quite close to the Altec 436 & possibly the RS124 both of which I have built. Therefore I can't speculate on the reasoning behind the component choices they made.
 

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