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Discussion Starter #1
hello again , finally got this 100 watt major cascaded build sounding good , got rid of the ghost note issue, and the oscillation issues . now have the 3rd issue high glowing tubes .... this amp has a dual mode pwr on .switch that gives the plates 540v/600v . im using kt 88s / custom merren ul o/t tranny and hammond pwr tranny modern g-lions . biasing around 35/40 mv.. when i play at 1/2 gain for a few minutes i see the tube really glowing , possibly heading for red plating but turn off asap... the voltage on the b+ is 540/550 . screen grid resistor was originally 220 ohm so I increased it it to 1k- 5w and it helped a little , should i go higher ? I also read on line that when dealing with high screen voltages you can up the screen resistors to 2k2 5watters to help lwr screen voltage, is this true ? there arealso 5k6 wire wound on the grids ... had this amp built last winter by a local guy , having nothing but problems ,..... any input will be appreciated jeff
 

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Yes you can go to 2K2 screens, but what is the idle voltage at the screens? And they are supplied by the UL tap of the OT or a node of the power supply?
 

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Wait this is a custom build by a local guy - take it back to him to fix if you've been having problems since taking ownership.

That said, in an UL amp you might not want the screen resistors that high.... and you're better off using carbons or at best metal film here not wirewound as a safety (better to blow the resistors). My Sunn 1200s had a screen resistor go and I was gonna upgrade them (value, type and power handling) but then I got the right info and didn't.

See this thread: https://guitarscanada.com/index.php?threads/1-of-4-screen-resistors-dead.81068/#post-728371

Most ultra linear amps don't have screen resistors at all. If they're there, it's not to limit current like pentode mode but rather to control oscillations and as a backup safety "fuse". The tap, usually 43%, on the output transformer limits current and reduces output impedance and distortion as well as giving a slight increase in power at the optimum tapping point, which varies with different tubes. Suggesting that higher value resistors would make the amp more controlled and useable nullifies the original designers' knowledge and intent.
That said a Marshall is not the same as a Sunn... but looking at Marshall Major schems online I see the screen resistors are 5W 250 Ohm on all of them, except 1 version which had 270 - insignificant difference (the plate resistor values vary much more by version) so the original value seems rightish assuming similar voltages (schems don't say so I dunno). Higher values may choke it out. The Screen resistors on my UL Sunn (also KT88s) are 47 Ohm 2W (but the voltages are a lot lower - 450ish). Your major is half the wattage of the original so consider lowering the rating of the screens to 2W as well.
 

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Discussion Starter #4 (Edited)
hey , i think i bought your solarus a while back, with the 6550's ...... as for this build , the builder and I dont speak anymore over this build because of the novice build at a high build price ... same day i picked the amp up it blew up output tubes in the 1st 5 min of playing , had some real bad ghosting issues which are now fixed . wires from output tranny were wired wrong , had that issue fixed ..I'm having a new schematic drawn up by nik at ceriatone , so i can start from scratch with new turret board and nos components . over the past year I had 3 tecks inside this thing and were pretty suprised to see such apoor build layout .the plate voltage is 540 b+\\ ;550 b+ and screen voltage across the 1k 5watt is little lwr[replaced the 220 ohm wirewound ] tubes didnt glow as bright with the 1k5w's .....I rewired some pre amp stuff with shielded cable to eliminate oscilating issues being that it is cascaded which was not done during original build ... [inputs to shared cathode on v1 ] ppimv wires , have 12ay7 in phase inverter, i find thatv3 tube gets quite , tried a 12at7 in there as well that gets very hot to touch as well ..i have chewed up 2 pairs of kt88 tubes tubes tying to isolate issues . now its the high voltages that got me stumped . I'M learning as I go here . if i could info on a step by step trouble shooting approach that would be great. THE PWR TRANNY IS A HAMMOND 278CX WITH VOLTAGE TAPS ON 100VAC AND 120VAC to give me the 540b+ and 600b+ , my original idea was to get it sounding as close to a real deal major in the 100 watt version . have to change out pwr tube sockets , as they seem loose and dont seat tight like they are supposed to , as per the builder he had ceramic ones , said that they were better ... I also noticed that pin 1 on these pwr tube sockets is missing , dosen't pin 1/8 get tied teogether and 1 ohm series to grnd .. does that hinder me from using like kt77's/ el34's with pin 1 missing ? also could the bias layout be wrong even tough i can get biasing ranges from 25/ 55 -mv ... when biasing around 32 to 35 the amp sounds great , when i bias 28-32 i notice the tubes get bright and amp output volume increases alot , if i bias upwards of 70% 43-45 it sounds really loose and undefined ..
 

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This thread reminds me why I stopped building/modding amps :) Probably best to gut it and start again as you mentioned
 
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