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On 8/26/2022 at 11:26 AM, DMH said:

I just realized that I also own this very nice JM Tercel! I've had this phono pre-amp for so long that i forgot all about it. 

It's also going to be checked out by the tech at the audio repair shop here in FW under the supervision of Mark. 

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@RealMarkDeneen I have another Juicy question on the Tercel if you do not mind.  I am going through my tubes and found an old post to me where you stated that in the Tercel the tubes are split with one side for each channel, so matching the two tubes is not necessary.

 

I have a pair of Mullards in the 12AT7 positions of my Tercel that I want to put in a different amp and a bunch of old singles of 12AT7s that I want to put in the Tercel.  I have the original Tercel 1 now (black face plate and green metal box). 

 

Which is Tube 1, Tube 2 (middle) and Tube 3?  I assume it goes front to back but I want to be sure. 

 

Thanks. 

 

 

Here is the post for you Tercel owners out there:

 

Ok, so each tube contains a Right and Left channel, and the signal runs in order from V1 to V2 to V3. The importance of the tubes is in exactly that order. V1 is the input and you want to make sure that it is the lowest noise possible 12AT7. I really like the RCAs lately, but a nice Tele is cool too. V2 is the gain tube and will highly influence tone. If you like it lean and clean and detailed go for a Bugle Boy here. If you like it smooth and dreamy, go for a Tele or Mullard. V3 is just the follower and has the smallest contribution to the sonics. Any nice 12AT7 is fine. I rarely put spendy tubes in this location, but of course if money is no object.....

If you want to get really fancy, get tubes where the two HALVES have been closely matched. Let me know what you end up liking!

 

 

 

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On 2/14/2023 at 11:12 AM, the real Duke Spinner said:

Why ..

Low Gain, High Gain, Low Gain ??

 

The way it works::

 

The 12AT is a higher current tube capable of driving a low impedance load. This is the input tube, and it is driving the passive RIAA EQ network, for which I like to use large value capacitors, which in turn means reltively low value resistors. This represent too difficult of a load to be properly driven by a 12AX. So, that's the reason for the 12AT input.

 

Then we need 40dB of gain, so the high gain high Z 12AX is used to get the most of the 40dB gain. So that's why the 12AX follows the 12AT. But we ain't done yet. We need a very low output impedance from the Tercel because we have no idea what it will be connected to in the way of a line stage. So, I opt for another Low output impedance tune with lot's of drive current - - another 12AT.

 

So, there you have it - - the basic" BBX zero feedback, high gain, passive RIAA EQ" phono preamp in a nutshell.

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On 2/13/2023 at 11:28 AM, tigerwoodKhorns said:

Mark,

 

Thanks for the response on the Tercel.  I have had both models. 

 

If I wanted to build a Merlin, how can I drive a subwoofer or two?  Is there a way to add a simple SS circuit to the design to drive an SS amp or two? 

 

I am not asking for a design, just a general question. 

 

Simple - - -don't build a Merlin, build a "Basic Peach". The Peach is  MADE TO ORDER for what you want to do. That SuperLoz Mode in Peach can drive a dozen SS amps at once. Yeah, it requires two more tubes than a Merlin, but that's no big deal. For what you want, that's the killer solution.

 

You don't need all the extras of the Peach. You just need the three tubes, One input and multiple outputs. No switch is needed for the various "modes" in the Peach. Just brute force it to the SuperLoz Mode.

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On 2/18/2023 at 6:21 PM, J M O N said:

Welcome back Mark!  Glad to have you back!

 

I do have a question for you that I've been wrestling with for the last few years and it is only because of my poor memory.  Do the pCATs put out 20 or 25 WPC?

 

 I think 20W @ 1% distortion, so it might be 25W before clipping. Something like that. There's no feedback so it clips very softly with rising distortion but not harsh clipping.

 

Glad to know they are working for you!

Cheers!

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On 2/15/2023 at 7:14 PM, RealMarkDeneen said:

 

The way it works::

 

The 12AT is a higher current tube capable of driving a low impedance load. This is the input tube, and it is driving the passive RIAA EQ network, for which I like to use large value capacitors, which in turn means reltively low value resistors. This represent too difficult of a load to be properly driven by a 12AX. So, that's the reason for the 12AT input.

 

Then we need 40dB of gain, so the high gain high Z 12AX is used to get the most of the 40dB gain. So that's why the 12AX follows the 12AT. But we ain't done yet. We need a very low output impedance from the Tercel because we have no idea what it will be connected to in the way of a line stage. So, I opt for another Low output impedance tune with lot's of drive current - - another 12AT.

 

So, there you have it - - the basic" BBX zero feedback, high gain, passive RIAA EQ" phono preamp in a nutshell.

Thanks for the kind reply, MD. 🙂

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I've got a question on a BBX Rev G(according to the tag) with the 6dj8. I turned it on this morning and nothing. No lights, no sound, no noises, no smells...nothing. The fuse is good. I looked around and all I could find is where it looks like the first green wire coming from the tx to the molex connector got hot.

 

Any thoughts or advise?

 

TIA!

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Hi--

Can you do a few voltage checks?

 

1. Check AC voltage across the two green wires. Should be around 20VAC or so. If not "bad transformer".

2. If that AC is ok, check the DC volts on top of the LM350. That is the large TO-3 device sitting in the middle of the 2" square heat sink on top of the PCB. That should be 6.3VDC +/- 1/2V. If NOT, you will need to replace the bridge rectifier which can be seen in the first photo. It is the 1" square black part near the long white resistor.

 

I will have to provide you the instructions and the part # you need to buy. It is the #1 most common failure in the BBX. Bad choice of parts on my part. However, once fixed it will last a long long time.

Email me with the results: aroadrunner@protonmail.com

Cheers

Mark

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Replacing the bridge rectifier on BBX

The next more common problem is the heater circuit BRIDGE RECTIFIER, and associated RC filter network. The rectifier is mounted directly on the PCB, and the spec was too low. They overheat and burn out. NOTE: if the bridge is burnt out, it might have taken the two large filter caps with it. Be alert for blown caps.

Do this:

--Locate the black plastic bridge rectifier – about 1.2” square near rear of PCB.

--Draw a diagram for your self noting the orientation and location of the PLUS, MINUS, and two AC pin locations.

--Desolder and remove the bad rectifier

--Remove all ceramic noise capacitors and throw them out, do not replace them

--Obtain at least a 25A 100V replacement bridge preferably with a metal base plate.

--Attach 4 color coded 14ga wires about 8” long to each of the 4 bridge lugs

--Drill a suitable hole through bottom of the Peach chassis near the PCB location where the old bridge was removed.

--Mount the bridge with a sturdy nut and bolt and washer directly to the chassis to act as a heat sink.

--Solder the 4 wires into the proper PLUS, MINUS, and AC holes of the PCB per your drawing.

--Test

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3 hours ago, RealMarkDeneen said:

Hi--

Can you do a few voltage checks?

 

1. Check AC voltage across the two green wires. Should be around 20VAC or so. If not "bad transformer".

2. If that AC is ok, check the DC volts on top of the LM350. That is the large TO-3 device sitting in the middle of the 2" square heat sink on top of the PCB. That should be 6.3VDC +/- 1/2V. If NOT, you will need to replace the bridge rectifier which can be seen in the first photo. It is the 1" square black part near the long white resistor.

 

I will have to provide you the instructions and the part # you need to buy. It is the #1 most common failure in the BBX. Bad choice of parts on my part. However, once fixed it will last a long long time.

Email me with the results: aroadrunner@protonmail.com

Cheers

Mark

Thanks for the reply and info. I'll take a few measurements and let you know.

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