Bellari RP220 Tube Mic Pre Modification

[Platefire]

I didn't know if I'm in the right place for this, so if this needs to be in solid state just switch it over.

This is modification I inherited from a friend when he felt it was over his head and wants me to finish it. He was doing the Black Lion Mod to a Bellari RP520/220 Micriphone Preamp . The night he was giving it to me the Black Lion mod documentation got wet and a lot of it faded out including the schematic. First I need to find a good copy of the modification plans to proceed. I've been searching but the best I've come up with is what I think is a RP 220 schematic. Also I've attached a scan of the one page describing this mod. If I could find the complete mod documentation would be a good start. I have a lot of the parts that came with it. Any help is appreciated. Plate

Edit: From what I find the Black Lion Audio site, no longer exist. I did find a Vodo lab page that might have the same info--Im looking QQ  
http://www.voodoo-labs.com/index/gearslutz_data/bellarimods.htm

[Platefire]

Trying to make heads or tails out of this. My friend that started this mod thinks his main mistake was the inccorect hookup of existing leads from the old input transformer that he reattached to new LUNDAHL LL1530 transformer that the Black Lion mod sheet recomends using for this mod. The Black Lion instruction sheet(pdf on first post) instructs you to refer to the attached LUNDEL spec sheet for hookup.

I'm having a hard time relating the amp schematic to the LUNDAHL spec sheet for hookup. On the bottom of the spec sheet it shows hookup and my friend was using the 1:3.5 layout. If anybody has time to look over my shoulders and maybe print out the schematic and the spec sheet and check my reasoning, I'll dance at your next wedding!!!! Refering to the spec sheet for 1:3.5 hookup I think:

           Primary side
#1 terminal showing the "+" is the grid from the xlr connector pin #2
#2 & 4 terminals are jumper, no outside connections required.
#3 terminal is from XLR pin#3  
           Secondary Side
#5 & 7 terminals are jumper, no outside connections required.
#6 terminal goes to Switch 2 for Phantom Power/DCV power supply
#8 terminal is grid going to R1 10K and to 1st stage of pre tube 12AX7.

Also I think E terminal needs to go to chassis ground.

Thanks, Plate  

[silverfox]

What is the purpose of the Mod? What is it supposed to improve or add to the product? I checked the page and it was kind of hard to follow the Mojo of the VooDoo due to the way the pages are written up.

Thanks,

Fox.

[Platefire]

silverfox

Thanks for looking! You must not have seen this(see attached pdf below) attachment on first post. This explains the reason for the mod in a general kind of way. They say it was a great mic preamp design hampered by cheap parts. So the mod is just changing resistors, op amps and transformers to better quality. My main concern right now it the input transformer hookup to get it correct. So I need to be sure I'm reading the hookup layout correctly before I change anything. The blue wires are jumpers with no outside connection.
  On the attached picture of transformer, the top is primary and bottom is secondary. From running continuity checks to trace the circuit I've found that the black wire is the + hot grid from pin# 2 of XLR input connector, the red is the - pin #3 of XLR input connector, the white is grid & goes to pin #2 of preamp tube and yellow is to the DCV power supply at switch #2.
  If I'm reading things correctly the primary side is hooked up correct but on the secondary side the white gird is going to secondary terminal #6, should be #8 and yellow power supply is going to #8 and should be #6. Also terminal "E" is a ground terminal and is not connected to anything.
Hope someone can verify my tracking on this one but if so you need to refer back to the schematic, transformer spec sheet and attached pix of the existing hookup. That may be too much to ask, but I'll give it a shot. Thanks, Plate  

[silverfox]

That clears it up.

"I'll dance at your next wedding!!!"

As for dancing at my next wedding- I can't image going there again even if the current one ended for some unforeseen reason. There is something to be said regarding the wisdom of Solomon and the 700 wives thang...

Thanks for the response,

Fox.

[Platefire]

That's just an old saying we have here in the south when your soliciting help which basicly means "I would appreciate your help". Don't read to much into it because I hope you and your wife are together forever--or death do you part. I still have my wife of my youth and plan to hang on to her. She's the only one that can put up with all my stuff  Plate

[PRR]

The schematic you posted has some very odd details, and may have errors. V1B has no ground return for DC cathode current. IC1 pin 3 has no DC path to ground/bias.

> was using the 1:3.5 layout

The sheet shows 1:7. And 1:7 is *much* more suitable for lowest-hiss from modern mike to tube.

1:3 might make sense if only LOUD sounds are involved.

E should *probably* go to the first stage's ground area; alternatively to the mike XLR ground pin. The old can should have had such a lead. Any connection will work in an electrically quiet environment. Harsh rooms may need experimentation.

Much of that list of suggestions seems dubious. Transformers, yes (I dont wanna think what junk Ross put in there). But wholesale resistor replacement seems like heavyweight work for lightweight effect. *I* would not condemn the 4560 opamp, it's good, though I suppose the '2134 is gooder. Replacing the pots won't change the "thermal noise", though if Ross used crap pots then with decent pots you get less crap noise.

I'm shocked they don't say to replace the low-bid tube??

If this helps, don't pack your dancing shoes.

[Willabe]

If this helps, don't pack your dancing shoes.

 

[Platefire]

I'll just dance, just forget next wedding!  

Thanks PRR, I can understand anybody avoiding this post and I feel completely out of my league
trying to figure it all out but I told my old friend Randy who owns/runs Snippa Creek Studio's in Alabama  http://www.randyandsherrimiller.com/snippa-creek-studios/  that I would at least take a look and see what I could do. This is a two channel preamp and this modification is just to one of the preamps. So the preamp on the right is complely stock and the one on the left is the moded one. He said he wanted to try the moded one first before doing the other.  
In reply to you comments:

1-The whole grounding scheme on this thing has got me bumfuzzeled! It would be so great to have the whole amp schematic and not just the preamp input to output. With the board sitting on the standoffs of the chassis, I can get no continuity from ground to chassis on any of the grounding points! Don't see any ground wires disconnected and can't even get continuity from one grounding point to another. I just discovered that yesterday and after spending about an hour trying to find some common grounding point put it up in frustration.
 
2-Thanks for the tip on the 1:7 hookup because Randy had started the mod and his hookup looks like 3.5--so I didn't know the difference and thought he might of had better info than I did.

3-On the XLR ground and all the XLR grounds, no continuity to chassis??? and can't find the common ground path that connects them. The old can has four leads and best I can tell, none of them are grounds? Like I said the right channel is all stock with the original input transformer to look at and observe.

4-It has the new pots on the left channel, old pots on the old channel. Some of the leads have already come loose that I will have to resolder.

5-On the the VoDo Lab site it says to use 5751's instead of 12AX7's?

Since we don't have a complete layout & schematic, would be glad to take some pictures showing all, if you would take a look. The grounding thing drive me nuts! Platefire

BTW-I also have Randy's Geoffrey Darking & Co SS 4 channel mic pre IV Version 2.7 that needs attention. I will eventually start a post in the SS section.

[terminalgs]

platefire,  your  pin number connection summary looks correct, to be clear, "#6 terminal goes to Switch 2 for Phantom Power/DCV power supply" isn't exactly correct.  #6 goes to the phantom power switch. the switch is a DPDT, phantom power is actually on the other side of that switch (top part on sch).  The side the original TR1 sec-7 is connected to is circuit ground for an LED indicator  for phantom power.   So connect the #6 secondary lead to ground somewhere, either ground at that switch, or close to the ground of the first gain stage (similar to PRR's recommendation to for the E connector).

That might help you find the circuit ground actually..

as far as chassis grounds go,  studio gear is has to be delicate when it comes to ground loops.  100's of patch cables, each device grounded differently,etc.   it is possible that chassis ground is not circuit ground at all.  look for ground-lift switches.  if the input and output are both transformer coupled, the circuit in between can just have a ground relative to itself.  chassis to circuit ground could be an AC coupling capacitor and resistor in parallel (or nothing at all??)

If your friend wants to take advantage of the 1:7,  a DPDT can switch between 1:7 and 1:3.5.  like PRR pointed out, 1:7 would be better for mike inputs, and 1:3.51 is better for line inputs.  Note the Lundahl title "mike transformer / DI transformer".  The 1:7 side is designed for mike, the 3.5:1 side is designed for DI.   In a DI application, Instead of the secondary going to to a
tube grid, it would connect to a 1/4" instrument level jack (thus providing the impedance match for inst. level : XLR line level). 

Does your friend use this as a mike preamp?  or just as a line-level preamp?

Also, R3, R5 and R63 seem counter productive for low-Z mike inputs,  SW3 is an attenuator, ....  what is the turn ratio of the original transformer?   I'm thinking its 1:7 or 1:8 ?  and the SW3 is (-)pad for  using the device with line inputs??

[terminalgs]

That U-pad of R3(4.7K) R5(1K) and R63(1K) was bugging me,   then I finally read the pdf you attached  say the Lundahl does not require a Zobel network in 1:7 configuration.   For either mic or line in, (1:3.5 or 1:7),  I'd either ditch that Zobel U-pad (which I don't understand R3 being 4.7K,  I thought a u-pad would be R3=R63 not R3>R63),  or better yet, ask the recording engineer what impedance devices does he plug into this thing. 

the only reason to leave that U-pad alone is if he has devices with greater than 600ohm outputs that he plugs into it.  and even then, he might have some inline XLR barrel type pad connectors to bring the device down to work with a 600ohm transformer.

[Platefire]

One thing, R63 doesn't exist. I looked and looked for that resistor and it's just not there on the board. Also I get continuity from xlr pin #3 of input jack to transformer pin# 3 red wire, if R63 was present, I wouldn't get continuity. I remember reading about the Zobel U-pad but I don't have a clue what that means. I'm sure the majority of what he will be plugging into it is various condinser mics. He has a lot of studio musicians tracking all kinds of instruments but I'll just have to ask. Platefire

[terminalgs]

One thing, R63 doesn't exist. I looked and looked for that resistor and it's just not there on the board. Also I get continuity from xlr pin #3 of input jack to transformer pin# 3 red wire, if R63 was present, I wouldn't get continuity.

All the better!!   

I remember reading about the Zobel U-pad but I don't have a clue what that means. I'm sure the majority of what he will be plugging into it is various condinser mics. He has a lot of studio musicians tracking all kinds of instruments but I'll just have to ask.

A pad is a voltage divider network used for (1) impedance matching and (2) attenuation. It does both.    the simplest is the L-pad, two resistors, unbalanced (like the volume pot in your guitar)  Two L-pad's mirrored on a balanced signal is a U-pad (three resistors).

this is good: http://www.uneeda-audio.com/pads/

[PRR]

> Zobel U-pad

A Zobel is not a pad.

As you know, the pad knocks-down all signal. It usually IS needed in a mike preamp which does harpschord one day and Screamin'Jay Hawkins the next. I don't get that drawing but I suspect the un-modded channel has a working pad.

A Zobel is an R-C network on the HIGH impedance side of a transformer which tames the high-frequency peak inherent in high-impedance audio windings. A 1:10 mike transformer may have 33K + 330pFd across the secondary. (You may also see 10K + 0.005uFd on the hi-Z primary of a cheap power amp OT.) This Lundahl is specified to give excellent result with no extra doo-dads hanging off it. The apparent 10K has no ill effect, and the main load is the 1Meg which is "unloaded" for practical purpose. So that's all good.

[Platefire]

Thanks, I'm learing a lot about mic pres through this. In order to proceed with the transformer hookup a couple of things I need to establish:

1-The previous moder was apparently taking the 1:3.5 hookup approch on the new transformer and to my understanding I need to re-wire to the 1:7 hookup to be in line with most usable/versetile type operation?

2-On the lack of grounding that I couldn't determine or find, I suppose this is normal?

3-On mounting the new transformer to board, the old transformer mounted with screws through board. The new transformer is larger and covers up the old holes when centered between the holes. My plan is to use a small piece of radio shack board sanwitched between the new transformer and board to provide insulation between ajacent resistor & board transformer connections and use a plastic tie through the inner
mounting hold and edge of board to strap transformer secure against board. This is the only logical mounting I could think of????

I'm wanting to move in a direction I can make new transformer solder connections/accomplish proper transformer mounting to board, correct existing broken solder connections to pots, partually assemble and do a operational test. I appreciate the help and discussion--without it I would be stuck in the mud!! This thing was given to me dis-assembled, so I hope I find all the board & chassis mounting screws. Thanks, Platefire

[terminalgs]

1-The previous moder was apparently taking the 1:3.5 hookup approch on the new transformer and to my understanding I need to re-wire to the 1:7 hookup to be in line with most usable/versetile type operation?

I would use a DPDT switch to provide 600ohm or 150ohm to the same XLR.

2-On the lack of grounding that I couldn't determine or find, I suppose this is normal?

Find the filter caps in the HT power supply, the (-) side of these should be your reference ground for the triodes.   

[PRR]

> 1:3.5 hookup ...the 1:7

Get it working. Either way. At this point the small difference of gain is the least of your hurdles.

I really don't know the grounding.

*Personally*, I dislike Rolls' designs. OTOH, I could possibly suss-out what they did and get it back together with the new parts. However I might get lost "improving" it. And on the third hand I might just refuse to look at it.

[smackoj]

FWIW I bought one Bellari mic preamp six or eight yrs ago and took it back for a refund a day or two later. That piece was so noisy that it was unusable for anything but a conversation piece on the coffee table.

Since that time I have had only good luck with the ART mic preamps which are inexpensive and the Ashley stuff is top notch also.

Maybe start with a better product and avoid the cussing and throwing hot solder?

 

[Platefire]

Har! My home studio mic pre is the little ART MP Studio with a 12AX7. Lowest of the line but for my needs is fine.

Talked to Randy yesterday and questioned him about why he made the hookup on the transformer he did. He said no rational reason, he just chose one and went with it.

So PRR says just hook it up some way to see if it works first. It will be a pleasant surprise if it does! Just looking at the Lundahl hookup sheet it also has a "3.5:1" connection I didn't notice before but I don't see hookup illustration for that, are am I missing something? Would you just switch connections on primary side for term # 1 & 3 for the 1:3.5 illustration? Platefire

[PRR]

> a "3.5:1" ...but I don't see hookup illustration for that

That PDF opens WAY too big for me. Shrunk-down there's a clear hookup for both options at the bottom:

[Platefire]

Yes PRR I saw those two diagrams but it has three combinations listed:
Connection 1:3.5
Connection 3.5:1
Connection 7:1

So 1:3.5 & 1:7 is shown as you copied on previous post, but where is 3.5:1? Platefire

[PRR]

I'm quite confused.

[Platefire]

Oh, I'm sorry PRR. Its one(middle one) of those three combinations of hookups on this attached pdf LUNDAHL sheet under "Reccommended termination for best square-wave response:"

It's talking about three combinations of hookups, first two in series with a cap and a third without a cap but there is only two hookup diagrams shown. Clearly I'm not comprehending what I'm reading and seeing here. It seems with three combinations of hookup, you would have three diagrams? I'm also wondering about the caps, I'm assuming they would be in series with the "+" grid, then would it be before input or after output of transformer?

This sheet seems to be geared to a professional that does this every day, I'm not that for sure! Plate

[sluckey]

I would think the transformer strapping would be the same for 1:3.5 or 3.5:1. The only diff would be which side you use for input and output.

[Platefire]

yep! Seems right to me. Do you have any recomendation which hookup would be best for mostly running condinser mics for vocals and acoustic instrument tracking. Platefire 

[PRR]

Ah. That's rather different.

Use the hook-up diagrams I posted.

The "Recommended termination" is a frill for users who listen with an oscilloscope. And it covers "backward" connections such as 3.5:1, which is about 10K:1K which might be useful for a Line To Mike conversion. Oddly it does not cover the 1:7 connection. I'm pretty sure it IS as good as it gets without hanging an R-C network on the hi-Z side. If we try to understand what it implies, it appears that for 1:7 we should use a 1K source on the low-Z side. We have 100-300 ohm sources. However we will NOT add resistance on the microphone side, because Hisss is more important than oscilloscope pictures. And I know these irons are very-good "naked".

[Platefire]

I'm waiting for inspiration to strike!   So far it hasn't. Might have to grab myself by the nap of the neck, just start soldering and then see what I got? If it somehow works good, it will be a Miricle . Plate   

[Platefire]

Uh Oh! Randy called me about this yesterday, guess inspiration has struck! Sometimes I think he is reading these post     ur hi Randy 

[Platefire]

Well I wired the transformer with the 1:7 hookup, put it back together and fired it up and all it produces is a loud hum. Platefire

[LHPcope]

Picking up on PRR's comment earlier, V1A also has no cathode ground point either, and neither of V1's grids have any DC ground connection (although there is a grid to cathode DC connection for V1B.)  As drawn, it looks like V1 produces no gain in the circuit, even though there are load resistors tied to B+ and sort of looks like it started out to be a cascode amp stage. 

As far as signal flow goes, the signal goes through R1/R28 then through R7/R33 then to the top of the level control with the wiper of that control tied to pin 5 of IC1.  The signal is also tied to IC1 pin 3 through C1/C12 from R1/R28.  These seem to be the main signal paths.

Can you inject signal on R1/R28 and get it through to the output of the preamp????

[Platefire]

  Well, this is my story and I'm sticking to it. I told my Friend that this preamp was outside my area of ability and knowledge but I'd have a look at it to see what I can do. This circuit is so radacally different from what I'm use to dealing with after you get past the basic hookup of the modification instructions, and it don't work, I could spend endless hours trying to get a circuit to work that is way beyond my comprehinsion. I've already spent a lot of hours on it already just trying to figure out how to proceed this far.

  Yes I have done some tracing of the circuit with my multimeter trying to locate a possible short between connections of the circuit but the way this circuit is built, I'm not sure I would recogonize the problem even if I ran accross it. Yes, if it was a simple point to point tube amp, I would know how to check it and finally pinpoint the problem. I'm about out of gas on this one!

Platefire

[PRR]

Pure hum suggests a missing signal ground connection.

You *know* what the tube pin voltages should be. Plate like a hundred or two volts. Cathode usually a volt or three. Grid normally dead-zero (+/-0.1V). Any radical difference, like +250V on a cathode, is A Clue. And HV on a LV pin again suggests a power-ground has gone AWOL.

I highly doubt it is a cascode. Seems like simple cascade of common-cathode stages, perHAPs with NFB from 2nd plate to first cathode (or maybe just a pot between stages (or maybe a mix of concepts)).

Agree that P2P would be easier to see.

Rip it all out and put in a Champ preamp. P2P of course. The transformer in front, and that opamp behind, make it a suitable studio mike-amp.

[Platefire]

Yeah, that's what I thought "bad ground" with the hum. I went around the circuit with alligator clips connecting chassis to different circuit ground points trying to effect the hum and get an output signal. Tried with mic connected through XLR input and also guitar through 1/4" input. I did get a slight distorted guitar sound when I put a jumper from gain pot ground to XLR output ground pin. Think I might of been jumping over part of the circuit to output doing that??

I think in a case like this you would need to find it with a signal generator and a listening amp and locate the breakdown in signal. If I presue it any further, might need to look a Dougs signal tracer amp project. I hate to give up on something even though it's over my head.

On the schematic I am a little bumfuzzeled with the terminal numbering on the pots. To my understanding standard pot terminal numbering looking at rear of pot opposite shaft, left term #1 is normally ground, center term #2 is normally output and right #3 is normally input. What's with the numbering on the schematic??? 1/3/2 showing center term as #3??? so now what terms are am I to assume is input and ground?? In otherwords I'm not sure the pots are hooked up right because of this numbering. Platefire

[LHPcope]

I highly doubt it is a cascode. Seems like simple cascade of common-cathode stages, perHAPs with NFB from 2nd plate to first cathode (or maybe just a pot between stages (or maybe a mix of concepts)).

   It looks like the cathode of V1A and the plate of V1B are in phase signal wise, so no NFB. V1 has no gain without DC cathode connections to ground reference.  So, is the schematic drawn wrong?  And if not what is the function of V1?

[PRR]

> So, is the schematic drawn wrong?

It's certainly more confusing than it needs to be.

It does not seem to be Authoritative.

You or I could probably poke a pencil at it and suss it out somewhat quickly. Depending how ugly the PCB is. Agree that without MUCH more documentation or information, Platefire could refuse the job without the least shame.

[LHPcope]

without MUCH more documentation or information, Platefire could refuse the job without the least shame.

I second that!

[Platefire]

Well this has got me considering a project I've been thinking about but haven't done yet. Doug's
listening amp as shown on this page: 

http://www.el34world.com/Hoffman/tools.htm

I have a 15 watt crate practice amp that would work. I could build the pre-coupeling cap & 1 meg pot in a metal box like Doug suggested. I've got plenty of MM extra test leads and sheilded cable. I wouldn't need a dummy load for this. Think I could just plug a mic in the xlr input and it would work the same way as plugging a guitar in.

With me working on tube amps pretty regular these days this is a needed project for future tube amp trouble shooting. Regarding the Dummy load resistor, I wonder where Doug got that resistor where you can adjust the Ohm resistance setting???

Platefire

[sluckey]

That's a very common style power resistor. Look on eBay.

[Platefire]

Yeah, seems all those variable ones are all NOS. Link to a e-bay 25 watter/100 Ohm variable was the closest I could find so far. Might be better to get a 100watt/fixed 8 Ohm? Plate

http://www.ebay.com/itm/171134647619?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_328wt_646

[sluckey]

Just keep looking. Something will turn up. Here's a link to a message thread about a 12Ω 300W tapped resistor. The eBay link is still alive but I doubt that guy has any more.

http://www.el34world.com/Forum/index.php?topic=13909.0

I built my dummy load using two 8Ω 50W resistors mounted on a big heatsink. With a single jumper I can select 8Ω/50W, 4Ω/100W, or 16Ω/100W.

[Platefire]

That's a good-un you built! Looks like that would control the heat a lot better than Doug's %>).

I've got an electronics tech friend that's also a guitar player and messes with tube amps. Everytime I talk to him about an amp I'm working on with hard to trace problem---he preaches signal generator and scope. Well this is not going that far but a step in that direction. I'll keep looking. Thanks, Plate