Dynacord DS-10 Microphone/Radio Preamplifier
« on: April 28, 2020, 04:49:29 PM »
Just picked up a Dynacord DS-10 preamplifier made by the Pentron Company in Chicago, IL in the 1950's. It's an all tube design with a single output, and a Radio (line-level?) and Microphone input both 1/4" inputs. I got it next to nothing and I've been wanting a tube preamp for random applications around the studio. My roommate fixes Fender tube-amps and I've got a couple years of building and fixing circuits, so I figured we could figure it out.

I wanted to ask the community though: Does anyone know anything about Dynacord? What about the preamps?

Apparently the unit passes signal only "part of the time" with a light noise. With it's age I'd imagine it only needs a cap job, new tubes, and a really good cleaning. The tubes in the preamp are as listed:

6BQ7 (x2)
6BQ7A
12AY7
12AU7
5Y3


If anyone has advice or information, advice on replacement tubes/parts, or opinions would love to hear about it.


Best,
Ryan



« Last Edit: April 28, 2020, 05:08:17 PM by FarisElek »


abbey road d enfer

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #1 on: April 28, 2020, 05:16:18 PM »
Have you noticed it's not just a preamplifier? It's the electronics of a tape recorder, including record and playback circuits, as well as an oscillator for erase and bias.
I would think signal passes when the "R" switch at the right is engaged.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #2 on: April 28, 2020, 05:19:21 PM »
I noticed the equalization for different tape speeds, but I didn't realize it contained all those circuits, no. Interesting. Thank you for pointing that out.
« Last Edit: April 28, 2020, 05:31:24 PM by FarisElek »

cyrano

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #3 on: April 28, 2020, 06:55:14 PM »
That's a Pentron Dynacord. Nothing to do with the German brand Dynacord.

http://museumofmagneticsoundrecording.org/ManufacturersPentron.html
Why is it people love to believe and hate to know?

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #4 on: May 08, 2020, 09:43:57 PM »
Okay so after giving it a few looks over, I'm a bit unsure of how to continue, although i'll probably just slowly replace things and try to leave it mostly as is in hopes I find the culprit. Basically, sound passes for about 8 seconds and then fades into static. Inside It's point to point so it's a little hectic. The tubes appear original and they all say "Made in Chicago." First things first, I'll replace the tubes. The filter caps are giant paper caps 500uf (mfd) and only rated to 60volts. I may try replacing those and some of the other paper caps. I'm going to check all the big resistors to make sure they haven't drifted too much.

There is this really weird long cable in the back that splits into two different XLR's that aren't really XLR's because they're too big to fit modern XLR's. It also splits into this four pin connector and two of it's prongs are connected with a GIANT wire. I'm not sure what it does yet, but if they wanted to connect those two pins, why didn't they just use a real small wire...or just bend the pins together and solder. This whole things is weird. It seems like whoever worked on it was more novice than I am...and that's saying something!

ruffrecords

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #5 on: May 09, 2020, 06:18:28 AM »
What you describe can be typical of a leaky inter-stage capacitor. As power  is applied the leaky capacitor gradually changes the bias on one of the tube until signal no longer passes . I suggest you replace all the capacitor connected to the grid of each tube.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

ruffrecords

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #6 on: May 09, 2020, 06:21:27 AM »
The flying leads are probably connections to an external tape deck. The ones with tick wires may be intended to switch mains voltages for capstan or take up motors or perhaps solenoids.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #7 on: May 09, 2020, 05:08:24 PM »
What you describe can be typical of a leaky inter-stage capacitor. As power  is applied the leaky capacitor gradually changes the bias on one of the tube until signal no longer passes . I suggest you replace all the capacitor connected to the grid of each tube.

Cheers

Ian


Amazing! I appreciate the insight,  that totally makes sense because a couple of the tubes are getting really hot quickly and some of them seem normal, which seemed like a bias problem.
« Last Edit: May 11, 2020, 04:47:28 PM by FarisElek »

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #8 on: May 12, 2020, 02:20:44 AM »
Is there an easy way to ground the psu and to make the inputs and outputs balanced?  For the psu: would I just replace it with a three prong and attach the ground wire to the chassis, right? And for the input and output would I have to wire in 1:1 transformers? Or is it possible to install trs or xlr using the existing transformer? I don’t understand why this has ts inputs and outs.

Ryan
« Last Edit: May 12, 2020, 02:31:46 AM by FarisElek »

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #9 on: May 12, 2020, 04:18:23 AM »
Good news!
I changed the tubes and the signal no longer dies. It just has no headroom and a lot of noise.
I'll be changing all the paper caps when I receive the new ones on Friday. I'm hoping this takes care of the noise, and that it fixes the bias.
If anyone has an other tips, it would be greatly appreciated.
I'm obsessed with the tube pre's capitol records used in the early 60's and I'm trying to get that tube saturated sound for bass guitar and other low end sources if possible.

Ryan


ruffrecords

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #10 on: May 12, 2020, 05:36:46 AM »
Is there an easy way to ground the psu and to make the inputs and outputs balanced?  For the psu: would I just replace it with a three prong and attach the ground wire to the chassis, right? And for the input and output would I have to wire in 1:1 transformers? Or is it possible to install trs or xlr using the existing transformer? I don’t understand why this has ts inputs and outs.

Ryan
You should be VERY CAREFUL if this device has only a two pronged mains inlet. First thing to do is check if there is any continuity between either of the existing prongs and the chassis. If there is none then it should be safe to connect the chassis to mains earth.

Difficult to comment on which transformers if any to use without sight of a schematic. It has TS connectors most likely because it is a domestic product.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #11 on: May 12, 2020, 03:32:40 PM »
Not getting any continuity between the two-prong plug and the chassis. I'm dying for a schematic for this thing. I'm not great with knowledge of tube circuits but I can surely read a schematic and research any questions I have. 

Does anyone have a recommendation to get the in's and outs of this guy to be balanced? Is there a transformer I can install into the ins and outs to convert balanced to unbalanced for in, and unbalanced to balanced for the out?

Ryan

abbey road d enfer

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #12 on: May 12, 2020, 05:47:17 PM »
Not getting any continuity between the two-prong plug and the chassis. I'm dying for a schematic for this thing. I'm not great with knowledge of tube circuits but I can surely read a schematic and research any questions I have. 

Does anyone have a recommendation to get the in's and outs of this guy to be balanced? Is there a transformer I can install into the ins and outs to convert balanced to unbalanced for in, and unbalanced to balanced for the out?

Ryan
In the absence of technical documents, I would suggest you try one of these
https://www.shure.com/en-US/products/accessories/a85f
or that
https://www.bhphotovideo.com/c/product/288115-REG/ART_CLEANBOXPRO_CLEANBOX_Bi_Directional_Level_Matching.html
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #13 on: May 19, 2020, 12:45:10 AM »
Ok here it is. Post cap-job/new tubes Pentron DS-10.

It's alive! Now that it's working I have some general questions I'm hoping y'all can help me with.

1) Is there substitute or recommended tubes to replace 6BQ7? There 4th one on the top from left to right is VERY microphonic. I've swapped it around with some of the other 6BQ7's I got and they're all bad and occasionally ring,

2) How do I check the bias on this? Does anyone know which pin is the heater?

3) Is it possible to install and output transformer easy? In the meantime and XLR to TS is working great on the way in. I'm running a reamp in reverse on the way out and it's the lowest noise set up so far. I can't run mono out into my converter, it's creating lots of noise that way.  The shure A85F removes lots of the clarity and also has some noise. I don't have another TS to XLR, but I'm assuming that it would work good on the way out?

Boy does this thing sound cool on guitar. I attached a talk test of the mic below. It sounds even better than on there since I stopped using the Shure A85F, but you get the idea.

Ryan

[url] https://soundcloud.com/ryan-virgilius-daniel/pentron-dyancord-ds-10 {/url]
« Last Edit: May 19, 2020, 04:34:03 AM by FarisElek »

abbey road d enfer

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #14 on: May 19, 2020, 06:11:40 AM »
1) Is there substitute or recommended tubes to replace 6BQ7?
I don't know a direct equivalent, except of course the European equivalent ECC180. You could probably replace it with a 12AU7, but you would need to rearrance the heater wiring.

Quote
2) How do I check the bias on this?
In the absence of schemo, I can only guess, measure the cathode voltage.

Quote
Does anyone know which pin is the heater?
4 & 5. You could have googled it...

Quote
3) Is it possible to install and output transformer easy?
Yes, space permitting.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #15 on: May 19, 2020, 06:27:50 PM »
Appreciate the help. I am a a total novice. I'm only good at soldering, following instructions, and selecting replacement parts. Everything else is a mystery to me. But yeah I could've looked up the datasheet on the tube, oops.

Can any recommend a great sounding output transformer?

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #16 on: May 23, 2020, 02:19:56 AM »
How would I got about install a ground on this. It seems that a hot goes to a switch that goes to a transformer, and neutral goes to a transformer. From the transformer there is 12ish wires. One of which goes to a 10 watt 25ohm resistor on the chassis. Is that resistor the "ground"? Should I connect the point where that wire is coming from to the resistor to the ground of the 3 prong? I'm definitely not going to proceed until I find a definitive answer so don't worry about me, I am very worried haha.

abbey road d enfer

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #17 on: May 23, 2020, 03:01:55 AM »
Is that resistor the "ground"?
certainly not!

Quote
Should I connect the point where that wire is coming from to the resistor to the ground of the 3 prong?
No!

Quote
I'm definitely not going to proceed until I find a definitive answer so don't worry about me, I am very worried haha.
You should attach the "earth" conductor to the lug that's bolted to the chassis. See attached.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #18 on: May 23, 2020, 03:28:41 AM »
and Hot and neutral should go to where they’re going? I don’t need to alter anything else? If I attach the plugs ground to that bolt, where can I find the circuit ground to attach to it? Or do I assume that circuit is grounded to the chassis somewhere.

Also, the 2 prong plug is reversible so I want to make sure I disconnect anything I need to disconnect I if I need to disconnect it.

Ryan
« Last Edit: May 23, 2020, 03:56:22 AM by FarisElek »

abbey road d enfer

Re: Dynacord DS-10 Microphone/Radio Preamplifier
« Reply #19 on: May 23, 2020, 04:08:26 AM »
and Hot and neutral should go to where they’re going?
Yes.

Quote
If I attach the plugs ground to that bolt, where can I find the circuit ground to attach to it? Or do I assume that circuit is grounded to the chassis somewhere.
It is.

Quote
Also, the 2 prong plug is reversible so I want to make sure I disconnect anything I need to disconnect I if I need to disconnect it.
In most of consumer gear, live and neutral are interchangeable. Don't fret.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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