One Electron SEA-1 Owners manual

SEA-1 – Triple 2A3 Amplifier

.01W

.1% 1% 10 %

.1W 1W 10 W

Harmonic Distortion vs Output Power

SEA-1 Amplifier

1000 Hz, 8 Ohm load, RCA 2A3's

General Description

he SEA-1 amplifier is a straightforward single-ended audio

T
power amplifier that delivers about 11 watts, using only

dium and low-mu triod

me
other than local cathod
or semiconducto
amplifie

r is characteristic of triodes: smooth and detaile d.

rs are used in the design. The sound of the

es. No negative fee

e degenerat

ion. No voltage regulation

Despite the lack of feedback, the bass is surprisingly solid.

his amplifier has been teste

T
no incompatib
need for relat

ilities have been f

ively efficient speakers (approx. 90 db/watt or

d on a variety of speak

ound, except, of course, the

better).
The SEA-1 was designe

T-1 transformer

UB
the SEA-1 was designed t
power suppl
could not be inc

y. There is no reason w

luded on the same chassis as the amplifier

long as hum-inducing comp

ransf ormers, ar

t
circuits.The s

upply needs to prov

and 6.3 volts AC at 1.8 Amps. Filt

hokes are incorpo

filter c
Dressen-Barnes mo

d to e valuate the prototypes of the

s. In order t

o provide flexibilit

o be used with a separate e

hy the power suppl

onents, such as po

e kept away fro

m the amplifie r driver

ide 395 volts DC at 160 ma,

ering is not critical, since

rated into the amplifier. A s

del 30180 “Unregulat

was used for each channel in the SEA-1 protot

citor or choke-input power supply

capa

RCA Rec

eiving Tube Manual

or Radiot

Handbook,can be used.

te that this amplifier description is intende d to aid

No

nced tube amplifier designers and builders.

experie
little experience b

uilding vacuum tube audio amplifiers should

not attempt to build this amplifier bas ed on just this

dback is used,

ed Power Suppl

ypes. A simple

, as described in the

ron Designer’s

ers, and

y in testing,

xternal

, as

wer

urplus

y”

People with

cription.

des

The voltages used in this amplifier are lethal!
Precautions for working on high voltage equipment must be
followed.

Measurements:

Frequency Response:10 Hz to 44.5 KHz (-3db at 1W)
Output powe

nsitivity:

Se
Damping F

All measurements were made into a low-inductance 8 ohm resistor
connected t

r and Harmonic Distortion: see graph below

22 db (to 8 ohm tap)

actor: 3.5

o the 8 ohm output tap.

Circuit Operation

Choke-capacito
driver stages. Se

y

uces feedback fr

red

r filters are used for b

parating the power s

om the output to the driver

insure the highest possible B+ v

cating the filter chokes on the amplifie

Lo
the power s
noise. Every e
capacito
capacitors whe

The input stage is a conv

upply helps isolate the amplifier from e

lectrolyt

r. R20 is a bleede

n powered-d

ic capacitor is parallele

r resistor to help drain the filte

own.

ent ional resistance-coupled
amplifier using the octal 6J5 or 6C5. The metal ver
preferred here,d

nvelope. The cathode is only partly b

the e

R3 and R4 was chosen to give an overall amplifier

of
sensitivity o

The se

cond driver stage, using par alleled se ct

ue to the electrical and magnetic shielding of

f 22 db.

6BL7GTA, has the difficult task of dr iv
capacitive inputs of the three 2A3’
to-peak.Despite the re

latively low plate resistor of 12.1K,

stage is still the limiting factor in the over

nse. An unbypassed cathode resistor reduc

respo
in this stage, but this raises the outpu
lowering the high freq

uency response. To compensate for
this, the small capacitor C8 was adde
and reduce phase shift at high frequencies. C1 ser
same purpose f
“Cathode Compe
amplifiers.C1 and C8 were c
flattest overall fr

The output stage is cathod
drop in the cathode resistor

or the first stage. This type of circuit is calle

nsation” and was develo

hosen experimentally to give the

equency and phase response.

e biased, with about 20 volts extra

s to allow a bias a djustment,

using R13.Medium and low frequencies are bypasse

oth the output and

upply filtering this way

oltage to driver tub

r chassis instead of

d with a film

ypassed. The ratio

ing the hig hly

s at up to 100 volts peak-

all high frequency

t drive impedance,

d to even the response

ped for video

, and helps

e V2.

xternal

r

sion is

ions of a

this

es distortion

ves the

d

d around

Application Note SA-1,rev 1.02 Pag

65 Washington Street, Suite 137

one electron

™

Santa Clara, California 95050 USA

© 1994 John A

y copied,as long as this copyrig

freel

twood,One Electron. This d

ocument may be

ht notice is included.

e 1