Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on July 26, 2014, 12:50:34 pm
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Oh Boy! I am a little confused on the actual date that this amp was made. Some research says 1951-55. I just saw one advertised as 1948.
Only one PS cap is dated 643.
Maybe that isnt a date code. Couldnt see any numbers on the volume / power on pot.
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If that's a date code it's either the 43rd week of 1946 or 1956 which obviously neither agrees with a 1951 amp. Just the style of the amp dates it to the era you mention though. I'm really curious to see some gut shots, I'll bet it's point to point since it's of the right time frame and such a simple amp.
If that's not a recover then it's in remarkable condition for it's age! How big is that speaker?
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It all original. (1) RCA 6SN7 , (2) RCA Gray Glass 6V6GT & (1) RCA 5Y3 rectifier. The speaker is a ROLA. I forgot to take good gut shots after i re-capped the power supply.
I have been messing up latley and getting lax in picture taking. Here are a few more pics's.
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Pretty cool!
Random comments:
Largish power transformer, no? Not by much, but seems on the larger side. That could easily result from Gibson buying an existing style/model versus ordering a custom-sized one.
From the size of the cabinet, I am a tad surprised to find this a push-pull 6V6 akin to a 5E3 Deluxe versus a SE 6V6 Champ/Harvard.
Love the shield between the 6V6 tubes, I bet that helps on re-entry, LOL.
What's the speaker size? 10"?
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From the book "Gibson Amplifiers 1933 - 2008" (Wallace Marx Jr.)
"The BR-9 was offered as part of the BR-9 lap steel student outfit. The first design of the BR-9 was also the last amplifier design from Barnes & Reinecke and is what most collectors think of as a BR-9. It has a trapezoidal case that is wider at the bottom than top, and it is covered with a cream colored leatherette. It also has a distinctive plastic grille-piece that was featured in a number of both Barnes & Reinecke sales materials as well as promotional materials for the then-new plastics industry."
BR-9, made from 1948-1954
They changed the cab design in 1950.
1x8" field coil speaker and an inter-stage transformer for the PI.
From the speaker grill yours is from '48 or '49.
'48 - 2,194
'49 - 1,860
'50 - 2,172
'51 - 1,744
'52 - 1,356
'53 - 1,410
'54 - 1,062
Total - 11,798
Brad :icon_biggrin:
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Great info. Thanks. The plate shield is actually between a 6V6 & 6SL7. 6V6's in the rear and 6SL7 front left and 5Y3 front right.
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The schematic for that (in Doug's library) is just great to look at. Talk about a low parts count!
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cool old amp. you're one lucky tech.
attempting replication would be pointless without the inter-stage xformer specs.
would you be kind enough to measure it? DCR of pri.; DCR of sec. start 2 end; DCR end 2 CT. then 1VAC to pri. and measure sec. VAC start 2 end? then approx size and thickness of laminations.
if you're uncomfortable doing, no worries and thanks for consideration.
--pete
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I'd put a grommet in the hole for those speaker wires. Lot's of B+ on those wires.
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The amp left yesterday. It sure did sound nice for a tiny thing. No further testing can be done
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The amp left yesterday. It sure did sound nice for a tiny thing. No further testing can be done
:sad2:
j/k :icon_biggrin:
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I have no idea why i didnt take gut pic's as the chassis sat there exposed. Thats a major part of documentation. Dah! :dontknow: :think1:
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As a starting point to reverse engineering the voice coil is the 1000 ohm labeling of the voice coil. IMO, most schematics of this era, listed , I believe the 1000 ohm is the DCR of the voice coil.
Wrong. That 1000Ω coil is the field coil, not the voice coil. There's a BIG difference.
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As a starting point to reverse engineering the voice coil is the 1000 ohm labeling of the voice coil. IMO, most schematics of this era, listed , I believe the 1000 ohm is the DCR of the voice coil.
Wrong. That 1000Ω coil is the field coil, not the voice coil. There's a BIG difference.
I'll be spitting out sand for next few days. Thanks for catching the error.
Well I know what you meant :). Yes typically manufacturers specify the coil resistance (err, impedance?) and also specify either a current or voltage that needs to flow through it to energize the coil for proper operation.
However, you can also vary the voltage and/or current into the field coil to modify the response of the speaker. Has an effect of the frequency response and sensitivity of the speaker. Some hi-fi guys use this trick to tune their field coil speakers to the room it's in. However, they are using the new very high dollar field coil speakers being manufactured today, not the old stuff like we are dealing with (although the same principles apply).
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Regarding I am not sure what year the standard changed on how field coils were specified. I have found old schematics that describe field coils using the DCR (Direct Current Resistance). Newer schematics state impedance.
So, the elephant in the room question is: what conditions do you determine and specify field coils. I believe Hammond uses different voltages, and frequencies to specify a lot of there O/T's. According to GE approach, you can replace an interstage transformer with two o/t's.
While others on the forum may differ in opinion, I believe you need a choke of 18 to 20 H, with a DCR of about 1000 ohms to replace the field coil.
I'd slap in one of the 500 Ohm, 16H chokes and be happy but otherwise I agree. The field coil is sort of neat in single-ended amps though, it can be used to buck hum that is present if installed with the correct polarity.
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In 1947, I bought a new BR9 amp and BR9 lap steel, my first store-bought steel guitar. Worked for a local farmer for two summers to pay for it.
I still have a few BR9 amp pieces from a swap I did a few years ago, like the chassis and the cab... and maybe the inter-stage transformer......
The one in the pics appears to be barely used...
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I can't supply any info on voltages etc. I'll look around for the inter stage transformer.......
Bill
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attempting replication would be pointless without the inter-stage xformer specs.
The book GE Approach, in Fig 6.7 describes the design of an interstage transformer.
The book is An Approach to Audio Frequency Amplifier Design, G. E. C. Valve and Electronics Department (download link (http://www.tubebooks.org/Books/GEC_approach.pdf), 1.9Mb).
Typically, a guitar amp interstage transformer takes the place of an electronic phase splitter and may provide some gain by having a step up ratio from primary to secondary.
Fig 6-7 from the book is for a 175-200w Class B2 amplifier(!). V1 is the split-load phase inverter, while V4 & V5 are push-pull KT66's. The outputs of the KT66's are at the cathodes, and feed your 1:1 coupling transformer, which is push-pull primary and push-pull secondary. The KT66 power tubes are necessary as drivers ahead of the coupling transformer, because the output tubes draw 20-25mA of grid current per tube at full power output.
Since the Gibson (and pretty much all other guitar amps with an interstage transformer) uses a single-ended primary to push-pull output (again, probably with a step-up from primary to secondary), and at sub-watt power levels to drive output tubes which do not draw grid current, the winding data for the GEC beast won't be helpful.
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I've swapped interstage transformers in in an old Gibby before. IIRC the ratio is something like 10:1 step up (center-tapped of course). I never found an exact match but whatever I did end up using worked fine. Just make sure the primary can handle whatever current you're pushing through it. This is usually the winding that fails on the older units (especially if the driver tube itself goes) although sometimes the smaller secondary windings break and you end up with just a weak sounding single-ended amp that crackles as the other side of the secondary tries to conduct.
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I don't think the ratio is ever "10:1" (which sounds like step-down to me).
For power tubes NOT drawing grid current, we can make good guesses.
The primary rated impedance must be greater than the driver tube plate impedance; if not, bass will suck. So 12AU7-like General Purpose (lower Mu) triodes of 7K Rp can use a 10K or 15K primary.
Step-up sounds like Free Gain, but there are limits to how high an impedance you can wind a wide-band audio transformer. Going over 10K will limit bandwidth (or/and cost much more). So 10K:40KCT or 15K:60KCT is a reasonable limit.
Third factor is that the primary must handle the DC current of the driver tube. And to give good drive into transformer losses, we'll usually work the driver much hotter than in an R-C-coupled stage. Maybe 5mA.
Such a rig can usually put much-much more signal on the grids than power pentodes ever need. (Good power triodes may need more.)
Yes, Hammond makes a pretty universal driver transformer for single GP triode to push-pull grids.
When you move on to power tubes that DO draw grid current, it becomes more work, because it is WORK to smack the grids positive, and the amount of work varies over the cycle. Generally the driver is a small Power tube, of Rp maybe 4K. The grids can get lower than 1K at peak swing. A step-down is needed. Unless the driver is way over-size, you need to get the ratio pretty darn close or you won't get full output. It starts to look like transformer-coupled *transistor* amp design. (I think only the Fender 300 went this way in a guitar amp; B2 amplifiers are more often found in huge radio rigs.)
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FWIW the IST I just bought for my build can be configed as 1:1 or 1:2(step-up), in reality I get a small loss at 1:1 and just above unity as a step-up - current configuration. It is only rated 10mA though.