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I have purchased a Newcomb 1625M TR player with 2-12" portable speakers,(Date Code 1961 Oxford 27 Watt Type) I have found this site very helpful for a novice like myself so I would like to help out by providing the schematic from the inner case to anyone who needs it,This unit sounds great when I plugged my guitar in to the mike input but I understand and would like to learn about mods to improve the unit for guitar use exclusively,if any one could give me a few tips I would appreciate it,PS I am good with a camera and will photo all of the circuts,chassis, etc before I undertake any mods, The Schematic in the case has shown 1 extra 12AX7 and 1 extra 6AN8 from my actual chassis? Thanks Tim
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That looks like an excellent donor amp.
The phono input is a 12AX7 which is a dual triode - two in one bottle, so that's why it appears as two separate halves on the schematic. Similar thing with the 6AN8, but it is a triode and a pentode, all under one roof.
Plugging into the mic input you go to a 6AV6; a great way to start the signal chain although with that high value plate load resistor, 560k, you've got a less than ideal impedance situation for guitar. So one opportunity is to study up on some small signal inputs and copy a known guitar circuit. Fender Harvard used a similar tube, for example. You could also design one from scratch after reading the Merlin Blencowe's excellent book.
A second opportunity, since you say that input does sound good, is to leave it as-is and go after the phono input, making a second input, voiced differently from the pentode, and giving you some versatility. You could use the 12AX7 as parallel, mismatched triodes for a nicely complex sound or try a cascade approach - one triode into the second, for more grind.
So many options, and so much reading and studying and building in your future!
RWoodLast edited by RWood; 03-15-2010, 08:40 PM. -
The 6AV6 is a duo-diode/triode. The diodes aren't used in this design, and the triode is similar to 1/2 of a 12AX7.-tb
"If you're the only person I irritate with my choice of words today I'll be surprised" Chuck H.Comment
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Thanks for keeping me honest - correction made. I was thinking of 6AU6.Comment
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Converting a Newcomb 1625 TR
Wow,,, Thanks for the options, I had not even thought about optional inputs, I am going to do a lot of listening and very little talking for this first project,so I really appreciate the advice,I have been reading about deleting or lowering the resistance value on the negative feedback circuit. Is this something I could toggle or footswitch to keep the best of both situations?Also do I interupt the feedback circuit before or after the external phono(RCA)jacks??... TimComment
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Originally posted by javelin View Post
I think you are referring to the feedback loop that goes through a 39k resistor to the cathode of 6AN8(A), and yes, you can play with the R value, put it on a switch, etc - it will affect the amount of gain you get. You can get fancy with a couple of resistors, a cap and a pot and make it into a Presence control. Look at some late tweed, early blonde circuits that Fender used. For starters, I would put that line on a SPST switch and listen to the difference it makes in vs out of the circuit.
The phono jacks and the Aux In, the External Amp.... if it was mine I would remove the wires to all that stuff and hard wire the .05uF cap that comes off of 12AX7(B) directly to the top of the volume pot. That is your signal chain and it's just asking to pick up unwanted noise, going through all that other stuff.
You want what is the 'phono in' to be a 1/4" jack, so perhaps you can use what is there to move some wires or move some jacks in order to accomplish that. Then you can plug in to that channel and listen to the sound of your guitar as it goes through the 12AX7. Make a recording and that will be your "before" sound, pre-modifictions to that input.
RWoodComment
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Converting a Newcomb 1625 Fun
Mr R Wood... Thank you once again , I am assuming that the current “Mike” input uses the 6AV6 as the pre amp tube? and the mod you suggest would allow me to have the option of utilizing the 12AX7 as the pre amp tube? If I understand this correctly I could have 4 different amplification circuits if combined with the switched feedback circuit modification.
#1… Mike input factory circuit
#2… Mike input modified feedback loop
#3… Phono input using 12AX7 pre amp tube
#4… Phono input modified feedback loop
If this assumption is correct I will hold at this point of modifications until I actually complete and test the result. I have attached several photos of the actual circuits, does anyone see any obvious component that has an early possibility of failure due to age (believed to be around 1961) or design? I am undertaking this adventure with only a very basic understanding of electronics because I have 3 sons age 11-14-16 who insist that tube amps are the only real amps and after buying them one ( Crate V16 )I thought we could all have fun finding out how and why they work. Indeed every word replied to this forum is now translated many times over into what part of the circuit you are talking about and what does it actually look like ( Schematic Vs actual Chassis wiring ) Just plugging a guitar into the “Mike” circuit and hearing the mojo of the 50 year old tubes
Mullard and RCA gave me a big grin and the kids the giggles….Once again thanks ever so much for the help and suggestions, we have much to do and learn…… Tim and FamilyComment
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Yes, that is correct. It is a single triode, as TBoy points out, so one gain stage from that input.Originally posted by javelin View Post
Yes, a second input. Your amplifier was built to have multiple inputs (mic, phono, aux) with two distinct channels. You will be using the one originally intended for the phono, which has an extra gain stage in that it uses both triodes from a 12AX7, for mo' better rockin' goodness. And all you need to do to play through it is wire the input to a 1/4" jack instead of what is probably an RCA (phono) jack. But maybe not - I couldn't tell from your pics.Originally posted by javelin View Post
You can look at it that way. I see it as two distinct inputs with a switchable NFB, but it doesn't much matter how I would say it, you ARE going to have four different sounds from which to choose.Originally posted by javelin View Post
I think that is a good ideaOriginally posted by javelin View Post
Usually amps of that age need their filter caps replaced, but you can decide that based on your ears, and whether you hear a prevalent 60Hz hum coming from the power amp, as it runs. Those would be the filter in the largish brown cardboard tube and the ones in the aluminum cylinder that protrudes up where the tubes are. If you hear no hum, don't worry about it just now. If you do, let us know and we'll help you do a filter cap job.Originally posted by javelin View Post
Aside from the filters, it is possible that some of the coupling caps (the smaller white Cornell Dubilier cylinders) are leaky and may ultimately get replaced. When you get a better understanding of their function, and how they block DC voltage from entering the following stage, you can test with a volt meter and see if there's a problem. If you are getting sound now, don't worry about them.
Your sons are very wise indeed. Just be careful when they explain to you that muscle cars are the only real cars.Originally posted by javelin View Post
Seriously, around five years ago I recapped an Ampeg Gemini with my son and it was a really fun and rewarding experience. We both learned a lot and spent time together. It's a great feeling when you turn it on and it works!
Good luck & keep soldering,
RWoodComment
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Did someone say Musclecars!!!
Your not going to believe this but they each have a Musclecar ready for when they have their drivers licence a 1969 AMX... A 1971 S/C 360 Hornet and a 1974 Javelin AMX I,m a novice at electronics but an old pro at rebuilding and racing restored AMC cars and yes the kids are learning that as well, If there's anything I can help anyone with in that area let me know I have owned 23 AMC cars over the years ( mostly Javelins) Cars are easy ,These old amps are hard to understand for me but I am a glutten for punishment and I want to prove to the kids they can learn to do anything they find interesting.... Thanks again for the support team... Ill post the next picture of the Newcomb project on one of the cars.....TimComment
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TR-1625
I picked up a TR-1625 (TR-1625m without phono preamp) at this mornings flea mkt. Have some questions that are relevant to Tim's. If you know the voltage at pin 1 of the 6AV6 (bias voltage- 0.84v in my case) and calculate what cathode resistor is needed for that voltage for the triode to be cathode biased, can you just remove the .01 input cap and replace the 18 Meg resistor with a 1M & add the calculated cathode resistor? Would this sound the same?
Is a switchable cathode resistor workable or to much noise when switching?
Maybe a 1.6v bias for more headroom & switch in a parallel resistor to get the .85v, hotter original? This may also allow some tone shaping with cathode bypass caps.
ThanksAttached FilesComment
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It looks like a 2 prong AC supply.
Add the 3rd prong if you haven't, as a first business to make sure the chassis is safe and grounded.
IEC plug...if you can find a place to fit the jack is a good option.Comment
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You're talking about converting it from a grid-leak bias to a cathode bias, and that is very do-able. You'll want to go all the way and change the plate load resistor to 100k, and then adjust the power supply dropping resistor prior to that final filter cap, to get the desired plate voltage.Originally posted by Tonewood View Post
Copy the rest of the input tube from a proven circuit. For instance a typical fender would have a 1k5 Rk with a 25uF bypass cap, 1M grid-to-ground as you said, and a grid stopper between the input jack and the grid of anywhere from 10k to 68k.
You can put the bypass cap, or a couple, on a switch and make it quiet by connecting a 100k resistor across each cap's leads. This helps with the charge/discharge noise. As for putting Rk's on a switch - I have never tried it so can't comment, but it would certainly be easy enough to find out first hand.
Btw, with that 7-pin input tube, you might also think about a 6AU6, which is a pentode. There's a great chapter in Merlin's book (linked above) on small-signal pentodes, and reading that will make you want to try one!
RWoodComment
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Newcomb 1625M guitar Amp Conversion
Well..... We finally found a moment to start the Newcomb 1625M conversion to a DIY guitar amp project.... our first adventure was cutting out the factory mike imput feedback circuit and installing a standard footswitch to enable in/out control as desired....This Mod turned out to be very simple once we found the 39K resistor which to looks easy on the schematic but took awhile to locate on the actual circuit board and had us looking up resistor color codes until it became obvious....The result is a move in the desired direction....
#1… Mike input factory circuit............ The guitar sounds OK but comes to life only at full volume with one of the two original 12 inch 1961 Oxford speakers...... as we don't know for sure how much effect the speaker is having on the break-up sound so we are going to try several 12 inch guitar speakers we have around the house and go with the best tone before further mods
#2… Mike input modified feedback loop ........ We wired in an on/off footswitch and this was our first actual " Mod" we now have the 39K resistor
in the feedback loop under our whims and desires.... yes complete control...
anyone in the family can turn this on or off as easy as a lightbulb..... we noticed the overall volume increased slightly but the sound broke up at a lower volume and the sound at full volume is approaching raunchy....
Next MOD Adventure
#3… Phono input using 12AX7 pre amp tube ...........TBA
#4… Phono input modified feedback loop.................TBA
I would just like to thank you Mr R Wood again for taking the time to simplify
this project into its basic sections where we as a family can have fun and learn at the same time, I would never have done anything other than plug into the Mic input and play guitar without your kind simplification of a possible next step which has encouraged us to step beyond our comfort zone of being spoon fed ( just buying tube amps)and started us on a journey where I can only hope my children learn to ask why and how in their lives and don't just take everything for granted because " that's the way it is"
Thanks again Tim.............. and KidsComment
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Different Models
I worked with the Newcomb 1625 when I was in the AV Club in high school more than four decades ago.
The 1625 came in two flavors. The regular 1625 and the 1625M. The latter had a GE Variable Reluctance
magnetic phono cartridge instead of the inferior EV plug-in ceramic. It had an extra 12AX7 to provide the
extra amplification required. I do not believe Newcomb made a different chassis, but rather populated it
according to the required configuration.
Although they have not be made for many years, the original GE cartridge and stylus can still be had.
However, although there are 0.7 mil styli available for the mono GE cartridge, it is rough on stereo records
because the GE cartridge has essentially no vertical compliance. If you wanted to replace it, a Shure M44C
is a good choice. Connect LG to R at the cartridge and connect the low tone arm lead to RG and the hi tone
lead to L. Readjust to the tracking force to 5 grams. It should sound alright. If it sounds alittle dull, change
the input resistor at the input to the preamp from 6.2K to 9.1K. The GE cartridge and Newcomb amplifier
relied on the cartridge's self-inductance in combination with the input resistor to provide the HF rolloff
portion of the RIAA curve. The Shure has a different self-inductance.
As for as using a guitar, the MIC input should work fine as is.Comment
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Instead of starting a new thread; just picked up one of these based in large part to this thread; it was used by a school and did not have attached speakers in the lid. Anyway it all seems good, put in 1/4 jacks for speakers and another to access the phono rca for using the 12ax7...it roars. But a quick question about the speaker tap switch. It seems to me there is a 1 speaker 8 ohm tap and a 4 ohm tap to use two 8 ohm speakers...is this correct?
Thanks
Tim
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