National Dobro Amplifier 6107A

[Diverted]

Hi all,

A friend of mine just found an untouched National Dobro amp, dates to about 1935. I was doing some research and I found a thread here which is pertinent because it addressed my only issue so far: The interstage transformer primary and secondary are both open, no good.
In the thread, Sluckey and others spoke about using a Hammond 124B transformer to substitute.

Here is that thread: https://el34world.com/Forum/index.php?topic=32871.0

Prior to me seeing the thread, I posted a question in an antique radios forum about my search for a substitute. A commenter suggested that Hammond transformer will not work without a lot of modifications that I do not feel qualified to attempt.
My main question is, who is correct? Is the consensus here that I can get away with the Hammond, or should I look elsewhere?

Here is the response I received on that other site:

Hammond make the 124 transformer in various form which is 10k:90k. And they sell just the wound bobbin, where a user might use their own laminations. The thing is though, in this application, the transformer has to supprt the DC current via the primary winding and the 124 types as they are , are not designed for that.

The way primary current is "supported" is either to have a large core cross sectional area or a type of core material that does not magnetically saturate easily (big transformer), or have a physical gap in the magnetic circuit. This is to prevent the core being pushed too far up its B-H curve and distortion and non linearity getting severe.

So you have a few options; you could check if the original laminations in your defective transformer will fit into the 124C bobbin and likely that would be ok. Or you could buy their finished 124A or 124B transformer, unstack it, and re-stack it as butt stacked with the E-I laminations together (not interleaved) and space those with a section of A4 paper. You would then have to check it with a frequency sweep injecting signal via a capacitor to the grid of the 56 tube and checking the amplitude response with the scope as you manually adjust the generator over the audio frequency spectrum, to check for satisfactory flat response.

[kagliostro]

Is the Dobro a PP amp ?

If so I don't see problems

The .PDF file datasheet of the 124B say: for PP use

https://share.google/RLSkaH6rkbowkZEVL

https://share.google/9ONw1reTO8qFo5Nj7

https://www.hammfg.com/electronics/transformers/audio/124.pdf

Franco

[passaloutre]

If the transformer can not support the DC current, you may be able to feed with with just an AC signal via a capacitor and use a separate resistive load like in this old Victor amp I have.

[Diverted]

If the transformer can not support the DC current, you may be able to feed with with just an AC signal via a capacitor and use a separate resistive load like in this old Victor amp I have.

Thank you for this! It jogged my memory about the same tip I read last week when I was first trying to learn about working with interstage transformers (see link). I really appreciate everyone's replies.

https://philcoradio.com/library/index.php/shop-talk/general/tips-on-audio-interstage-transformers/

[kagliostro]

Note C23 on the schematic

Franco

[passaloutre]

I’m sure the coupling capacitor value and the transformer inductance have a strong effect on the tone, so you may have to break out the LC filter calculator.

[Diverted]

I’m sure the coupling capacitor value and the transformer inductance have a strong effect on the tone, so you may have to break out the LC filter calculator.

I have exactly zero experience using an LC filter calculator. Any tips on how to use it?
I may just play around with the cap value if it sounds thin, or thick, etc. I'll try to use a resistor that gets the 56 plate voltage down to recommended spec in the data sheets and go from there. Thanks!

[kagliostro]

Te larger is the coupler cap value the larger is the bandwidth

The capacitor (\(C\)) and the circuit's load impedance (\(R\)) form a high-pass filter. The cutoff frequency, where the signal begins to attenuate, is calculated with this formula:\(f_{c} = \frac{1}{2 \pi R C}\)Larger Cap Value: Lowers the \(f_{c}\), extending the bandwidth into deeper bass or lower frequencies.Smaller Cap Value: Raises the \(f_{c}\), limiting the low-frequency bandwidth and filtering out deep rumble or low-end noise.

C23 on the schematic has a value of .1

Franco

[passaloutre]

On second thought, the 124B transformer is expecting a balanced signal (one that has already been split by a phase inverter). If you use a 126 transformer (https://www.hammfg.com/electronics/transformers/audio/126) you wouldn't have to make any changes to the circuit, as these transformers are expecting a single-ended signal. Unfortunately they are a bit more expensive.

Nevermind all that, the 124B does not have a balanced input.

Looks like the 124B has a 65H inductance on the primary. Feeding it with a 0.1uf coupling capacitor (like in the Victor amp I linked earlier) will give you 62Hz hi-pass filter, low enough to cover the guitar bandwidth.

https://www.omnicalculator.com/physics/lc-filter#calculating-lc-high-pass-filter