Direct out for Ampeg B15 style build a la REDDI

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emi2345

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Joined
Oct 20, 2017
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46
I built a version of the Ampeg B15 a year or so ago and included the preamp out/power amp in jack socket. I can hang a JDI box off it but I was wondering if I could improve on that by adding some sort of tube buffer and transformer to it, to get a lovely tube DI.

Possible options are: 6SN7 in parallel AC coupled cathode follower with a 1:1 or 2:1 line transformer, some sort of SRPP or E88CC in parallel common cathode gain coupled to a 7:1 transformer like the EMI REDD .47 circuit.

Schematic:
https://www.manualslib.com/manual/447560/Ampeg-Heritage-B-15.html?page=10#manual

other images
https://www.dropbox.com/s/acv2fionmsdwvgy/IMG_3381.JPG?dl=0
https://www.dropbox.com/s/7covihbhv26vfaw/IMG_3121.JPG?dl=0
 

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Nice Work Emi,
The tube B-15 is such a classic .
I think going to all the trouble to create a Di out from the amp is not nessesary ,the output stage is where the magic happens in a B-15 .
Putting small signal transformers into a big tube amp like that your probably going to end up fighting induced hum from the mains transformer.

If you want to record from the speaker output silently then you need a big resistor to take the place of the speaker
If you were crafty about it ,you could  lift the speaker negative from ground , centre tap two 4 ohm resistors to ground ,and then another resistive attenuator to drop the levels a bit and you could take a balanced output direct to your recorder inputs  so your getting all that glorious low end funk throb from the output transformer saturating .
 
got a B-15 with a Reddi DI sittin at home,

used the 6N1P and 3.77:1 xfmr, took the signal right off the input jack with a resistor,

XLR output,

the Reddi has a wonderful sound of it's own, and coming off the input jack keeps the noise down,  killer setup for live,

and it has a vol knob in case the sound guy tries to short change you,  or the drummer drinks too many Green Monsters,  ;D
 
Thanks for the help guys! I tried a REDDI once, it sounded great. Confusing name though, doesn't have much to do with EMI REDD as far as I can tell (or the B15). To make matters more confusing, I like the idea of using the output of the REDD .47 preamp as a driver, taking the signal from where the ext amp is in the circuit . Ian (ruffrecords) has given me lots of great advice about it here: https://groupdiy.com/index.php?topic=67537.20. One problem is I don't see where I can fit the Sowter 9980 transformer (it is 45mm diameter and my chassis is only 35mm high internally). It has a mumetal can, is that enough to shield from the other transformers? My chassis is steel. Can I grommet mount it to the back or put it on top between the output transformer and choke?

Alternatively could I use the 1:4 Sowter 8540 instead? It is 24mm tall so would fit

http://www.sowter.co.uk/specs/8540.htm

Thanks,

Edmund
 

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I have tried this three ways.  I worked an Edcor transformer 10k to 600 for all three methods.

The first was to essentially hang the Edcor off the traditional "extension amp" and I think I may have beefed up the value of that coupling cap.  That worked Ok.

I rewired it with a pot, a resistor and the same transformer off of the speaker jack.  Used a 1k pot and I think I messed with the resistor value until the voltage divider gave me 0db at 12 o'clock coming out of the transformer into 600 ohm.  I can't remember the exact resistor value, but it was high enough that the Portaflex output transformer wasn't bothered by it.  This method was less pure clean sine wave, and more like what the bassist was hearing. 

At some point I tried jiggering the voltage divider to come up with a nice -30db and wired the xlr to the secondary center tap so it was theoretically closer to mike level and more of a DI. Running a P-Bass through 4 or so tubes and then knocking the signal down only to bring it back up meant that it would bring up the noise of the tubes more than I liked.  It worked, but compared to the line level output it was noisy.

If your iron has a numetal can it should be fine outside the chassis.
 
Big fan of classicly styled builds like yours ...  and with an added proper line amp, it's killer in the studio.

Provided you can keep the psu clean and the wiring quiet, its a great addition on-chassis.

Nice work  8)
 
One issue there is that the signal from the Ext. Amp. socket is post-volume so, for DI-only use you still need to load the amp. If you want to make silent recording, you need a dummy load.
An alternative is to disconnect B+ on the power amp section only.
Indeed, if you want to use simultaneously DI and mic, ignore my comment.
 
I was thinking about having a switch to choose the signal to the ECC88+transformer circuit either ’passive’ from the input jack or ‘active’ from  the preamp out position. The 6SL7 triodes have a gain of about 50 each (there are two per preamp channel) and the James eq has a loss of about 12db. Would this work and how would the level of the two options compare?
 
> The 6SL7 triodes have a gain of about 50 each (there are two per preamp channel) and the James eq has a loss of about 12db.

The B15 plan in the first post has unbypassed cathodes, gain more like 20-25 per stage.

The James is usually 20dB or 10:1 loss.

> how would the level of the two options compare?

I don't see an actual Plan here, just ideas and pointers to other stuff.

Draw it out. Think. You may be able to answer your own questions.
 
might be some tidbits here>

https://groupdiy.com/index.php?topic=56536.0

semi relational>

https://groupdiy.com/index.php?topic=59823.msg759677#msg759677

we built a DI output transformer for a 12AX7a, 4:1 on a Neve core a while back , cap input so no DC on the Nickel core,
sounds cool and cools off the output level,




and just for kicks, the Alembic  Garcia box, just add Mac 70's , 3 JBL,s for weird impedance, 5,000 hits of acid and  a Wolf guitar and your there>
 

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Straight out of the bottle CJ, now your talking...

Except I'm past it for that sort of behaviour... ;D

Still nice pre amp dude
 
CJ said:
might be some tidbits here>

https://groupdiy.com/index.php?topic=56536.0

semi relational>

https://groupdiy.com/index.php?topic=59823.msg759677#msg759677

we built a DI output transformer for a 12AX7a, 4:1 on a Neve core a while back , cap input so no DC on the Nickel core,
sounds cool and cools off the output level,




and just for kicks, the Alembic  Garcia box, just add Mac 70's , 3 JBL,s for weird impedance, 5,000 hits of acid and  a Wolf guitar and your there>

Sorry, what is the B+ voltage? Ia m in the UK so the transformer has to be 230-240V.  any details about the one you have used?

Thanks
 
OK I drew out a schematic for what I'm talking about. I've already built the preamp part, we're just talking about the E88CC DI section (I also haven't implemented the channel cascade switch but have the parts to try this out too).

Any suggestions as to whether this is worth trying or a bad idea?

According to Brain Sowter:

9980 will handle +35 at 50 Hz (high Z side).
8540 will handle +29 at 50 Hz (high Z side)

I would prefer to use the 8540 as it will fit inside the 1U high enclosure. Will it be able to handle the level?
 

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emi2345 said:
Will it be able to handle the level?
You shouldn'( really worry much about it; in the "passive" position, you are not very likely to see more than 0dBu, even from an active bass, and in the "active" position, you have a level control that prevents any overload, and anyway, the output from the second triode is not likely to exceed +30dBu in its comfort zone.
The -18dB pad doesn't compute with R31+R41totalling 800k and R42 at 75k; that looks more like 21 dB.
 
Thanks abbey road. I might see how the circuit performs before I put a pad in.

I moved C14 slightly from its position on the REDD.47 schematic, it used to be from R34/35 to R38. I did this to keep DC from getting upstream onto the grid of the phase inverter, will it change the sound/performance of the circuit at all?
 
emi2345 said:
Thanks abbey road. I might see how the circuit performs before I put a pad in.

I moved C14 slightly from its position on the REDD.47 schematic, it used to be from R34/35 to R38. I did this to keep DC from getting upstream onto the grid of the phase inverter, will it change the sound/performance of the circuit at all?
Any change would be the result of some parasitics, which cannot really affect the response significantly.
 
The plate coupling cap on the line amp seems quite low ?  Won't this roll off lfreq significantly ?

I use around 1uF in my redd style line amps usually, and even higher in other parafeed line amps.

Also, the preamp plate caps seem similarly a bit low to me, although I guess it's part of the ampeg b15 signature sound.

And 450K plate load on the sl7 seems very high. Great for psrr though!  I mostly like to to run a 6sl7 with a decent amount of current, and 150K or so plate load max.

The only time I've used 470K plate load has been with ax7 in a hifi  integrated amp's preamp section, that had a lot of nfb into the cathode. It really needed that extra bit of gain to cater for phono/riaa/nab curve stuff.

Notwithstanding, I'm sure the ampeg guys had their reasons ... no disrespect meant!
...

Interested in how it all shapes and measures up  ....    I'm reworking my own 'tube amp + line stage'  build to really hammer hum down as far as I can, given the modest amount of chassis space. 

I have about 2/3 of the area of a 19" rack and keeping sensitives away from high amplitudes is challenging - some heroics required  ...  better seperation of wiring,  and for the preamp/lineamp, a second,  high L choke (Hammond 157G  30H@40mA) plus dc heaters to boot.

I tend to overdo it on R-C psu filtering stages - I'm going for a pair of 3-section JJ electro caps for my rework. Something like 120uF total for the power amp and around 270uF total for the other stages.

As always, it's that last couple of dBs of noise performance (eg. 3 to 4 dB or so) which is hardest to achieve, and delivers the difference between 'working' and 'inspiring'.

I'm using a utc a-44 traffo with the optional shield, and it still picks up hum pretty good. One has to get the right utc orientation to even stand a chance of good performance . If I would redo the money, it would be some mu-metal canned Sowters!

Even after all that effort,  to try to achieve something approaching 80dB of signal-hum for the line amp, at +4 dBu nominal,  with a happy +12dB or so  of headroom ...  when the power section is doing 10Wrms or so  ...  I still use a gate-compressor 1ru unit off the line amp .....  to really get where I want to be ...  guitar amp recording fidelity wise  ;D 

For cheaps, I like the ART TCS unit! A few dinares more for a dbx 1066 would be a worthwhile refinement. I used to use my Ymaha 03D digital mixer for it's excellent 'compander' section to similar effect. Ultimate gate imho is the TC Finalizer+  .. which I have on the main mix  bus  8) 

I *am* finding that modern gate plug-ins are really starting to shine  ..  for use 'after-the-performance' - like the Eventide gate plug, or the SSL native or TeamDNR's MixControlPlus channel strip plugs.

A good, low noise feed provides that extra bit of long duration, low dB  'tail'    for downstream gates to work with.

...

The combination of low-ish power (10 to 15W) tube combo with  a great line stage is dynamite in a 'recording' context  - its really thrilling to capture a true-to-life sounding feed of the amp and re-inforcing that thru the monitor/fx/record chain,  to really fill a room at reasonable volumes  .. where 'lounge room' style tracking captures the most inspired performances.

I like to run a few mics as well ..  for blending options inside Cubase  :)  Pure luxury  8) 8)
 
I fitted the E88CC circuit and the Sowter 8540 4:1 transformer and I am extremely happy with it. It is very quiet with only the tiniest hiss from the tube itself, no hum at all when I was trying the Sowter transformer in different positions, it ended up directly underneath the output transformer, the mumetal shielding is very effective. I have the 'active/passive' switch installed (which switches between taking the DI input signal straight from the input jack or from the output of the preamp) and in both settings the level is good, I think I prefer the 'passive' setting into a mic preamp with a little gain. With a 30 ohm output impedance we can drive a 2k mic input as well as a 10k line input :) Also the 'active' setting reduced the amplifier output volume a bit, I figured that must some of the local negative feedback from the E88CC stage getting into the phase inverter, so I put a 470K resistor between the preamp out and the active/passive switch which reduced this volume change to a negligible amount. The only potential danger is if I engage the cascade switch which cascades the two preamp channels, we can get a nice gnarly overdrive by opening up the first volume pot and using the second volume pot as a 'master'. The level can get very high very easily and we could put some serious volts into whatever the XLR out is plugged into.

Thanks everyone for all your help and input :)
 

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