Any Moto heads? Need OT advice please - diyAudio
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Old 14th April 2012, 02:50 PM   #1
diyAudio Member
Join Date: Oct 2011
Default Any Moto heads? Need OT advice please

I have a Motorola HS-711B console (16w)(4-EL84's and 12AXTx2 with a 5U4G) I got with no output transformers? I'm going to re-cap and do a full resistor R&R and make it a stand alone. It has a huge PT. The orginal OT's were from the schematics 5500 Ohm input and 6-8 ohm out.
I thought I might get a better FQ responce and sound by taking the coulping caps up from .02MFD to say 1.5 or .22 and putting in a pair of new OT's-These are the two I am looking at-suggestions?
input-7.6K ohm
20-20K Hz responce

input 10K ohm -it would lower the gain by about 1dB
out-8 ohm

I also thought about doing a UL Mod by going to pin 6 on the EL84's with a in-line 100 Ohm 2w resistor from the OT's?

Your thought are appreciated and I would inc the schematics but I get a "File size to large".
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Old 14th April 2012, 05:39 PM   #2
DF96 is offline DF96  England
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I assume this has no NFB, or you understand and have modelled the LF stability effects of a change in OPT and coupling capacitors? 'UL Mod' probably requires more than adding a resistor.
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Old 14th April 2012, 07:46 PM   #3
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It does have NFB-I think I will go with some 12AT7's instead of the 12AX7's and go Ul using a pair of OT's with these specs then
10K input
6 ohms out
40% s/g
40-18k Hz

Last edited by oldtexasdog; 14th April 2012 at 08:12 PM.
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Old 15th April 2012, 01:01 AM   #4
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Also,I'd be careful about your proposed coupling cap values. I'd start with double the 'stock' value(0.047uf),and go from there. Beware that too large of a value may cause LF stability problems,.22 might be a bit on the large side. 1.5uf would be insanely huge,and is likely to cause problems,IMO.
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Old 15th April 2012, 01:36 AM   #5
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Thank you!
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